Level 12 Analysis

Your AC is very low. At level 12, AC 18 is essentially a death sentence. I'm going to assume you have a source of Mage Armour from a party spellcaster, giving you an AC of 22, and a further Ring of Protection or Amulet of Natural Armour or Barkskin or something to up that by +2 to 24. 24 AC is still incredibly low.

The lowest attack bonus I found with a quick peruse of CR 12 monsters is +18, the secondary Bite of an Athach. That hits you on a roll of 6. The most common number I found was +23, which would hit you on a 2. I saw lots of +26, +28 and even a +33. Those all hit you on everything but a natural 1.

In other words, your AC is too low if anything targets you for you to survive. The DM will have to pull punches not to kill you.

Valiant Stand improves this. By not by enough. Enemies are still going to autohit you. Best case is +4 stat boosters on dex and wis, mage armour, nat armour/deflection bonuses, and valiant stand - 18 base, +4 from enhancement to stats, +4 armour, +2-+3 nat armour/def, +3 valiant stand = 32. That's a hit on a 9 from the lowest common attack bonus I found on CR 12 monsters.

Your Attack Bonus Is Pretty Decent, For a Monk. You have +16 to hit without Smite. With Amulet of Mighty Fists +2, that's +18ish to hit. With Belt of giant Strength +4 or Bull's Strength, that's +20 to hit. Everything that has AC that is supposed to be a defense has 26+, which is good - you hit most of the time. Power Attack and Flurry are counterbalanced by Valiant Stand, which you should be endeavouring to have always active by use of Stealth, which you don't have maxed out, uh, why? Popping up out of nowhere for a flatfooted smiting power attacking flurry should be your opening round action in every combat. Even your iteratives have a decent chance to hit, with this hit bonus. Only decent, though.

Your Damage Isn't Great. 1d10+1d4+5 (where is that errant +1 damage coming from?) averages out to 13 points of damage per strike. With Power Attack, that's 19 damage. With Smite, against evil or chaotic outsiders/dragons, you get /+18/, so 34 damage per strike, which is actually level 12-worthy. Too bad you get 3 total attacks per day with that damage. Sweeping Smite carries the Cha-to-hit bonus, but not the damage from the 'first hit'. It's kind of like a really terrible whirlwind attack that way.

Most of the time you'll be attacking for 19, on average. Some stuff is immune to bleed, or has DR/cold iron and good, etc, so it might be less. If you move, you only have one attack, so your first round barring stealth will nearly always be one attack at best. Rounds after that, you can flurry for four attacks, each which might hit for 19 damage.

So a max damage on an ideal round of around 34+19+19+19 damage, or 91 damage in 4 hits. That's not terrible. It relies on getting a full round action, though. It also relies on hitting with iteratives, DR of the kind you can get past, no miss chances etc. Most monsters at this level have at least 150hp, so you won't be winning in hp races. But you will be able to contribute a bit at least to the usual team of four on one beatdowns.

Your HP gets eaten in a round. With your AC, most CR9+ melee monsters are going to kill you to dead in a round. 11 Constitution on a melee combatant isn't a great idea, but then, you're a monk and need wis and and dex and str and also cha for being a paladin, so eh.

Your Saves are decent. They could be better, but they could also be worse. Most DCs seem to hover around DC 23 at this level, going up as high as 28 but also as low as 21. With your +11/+11/+13, you should have a 50% shot at most saving throws, and although fear and poison aren't exactly the most common life-threatening things to save against, it's still nice.

Power Level: Fighter.

This character can be relied upon to be roughly as good as an unoptimized fighter. The DM will have to pull some punches, and tough encounters (CR +2 or greater) will have a good chance of one-shotting this character by accident. In a party with Rogue or better power levels, this character will feel notably weaker than the rest of the group.

Note

Evaluating something without gear is hard, because at the lower end of the scale, gear can have more effect on your ultimate spread of numbers than the classes chosen. If your gear is random stuff, you will be weaker. If you plug it all into boosters for your FOUR necessary stats (dex, wis, str, cha), rings of deflection and other stuff that helps boost your melee stats to make you 'great' at that rather than 'mediocre to good', which includes buffs from friendly spellcasters, you will be stronger.

Note

This build is stronger at lower levels, where a handy Mage Armour plus your defensive stats can put you out of reach of opponents attack bonuses, and things are easier to hit and easier to kill. Level 12 is definitely where things are starting to go 'downhill', and they will likely get worse as enemy AC and AB increase and every enemy has flying or spells and abilities.

Note

The build has no movement capability other than skills-based climbing, swimming, jumping. I'm assuming again allied spellcasters will be patching that hole with buffs, especially since with the other gear i've assumed in your build you don't have the money to buy gear to get flight or teleport or whatnot.

Suggestions

Your main problem is that you're playing a monk. Without serious optimization, monks are very weak. You've done a bit of optimization here, and pushed it up to unoptimized fighter level, but fighter is still not great if you're planning to be in a party with casters. Or if you're fighting caster-style enemies like many demons are.

Most of the tricks I know to make melee characters more competitive are in 3.5e. I'm assuming there's no backward compatibility in your group. So that's all out the window.

Paladin isn't giving you much other than flavour, either. You've got a decent Flurry and Attack Bonus, but you're way too fragile to make good use of it, unless your enemies come in the 'always attack the guy using the full defense action with the tower shield' variety.

In this case, i'd recommend Summoner. Synthesist Summoner. It slices, it dices, it buffs, it has level appropriate abilities, natural attacks, the works. It is, in fact, so good that you could mix it with a Paladin dip and still be relatively awesome.

Evangelist lets you keep advancing as a Summoner while also adding a divine feel to the class. It's also the only prestige I can find that advances your eidolon aka living armour, so.

Other. than. that... there's a good rogue-based fear build, but I don't know how relevant that is to your concept.

I feel like there's some way, between Fighter archetypes and the Hungry Ghost Monk monk archetype, to create kama-wielder who reaps ki from his opponents and uses it to chain-stun them with a huge Wisdom and Dex, just whittles them away with flurries of weak, ki-reaping attacks and then spends the ki on stuns. There's too many damned archetypes though.

Even if you're taking summoner, it's kind of worth dipping monk as well. You have to be unarmoured to use your synthesism, so you could benefit from a high wisdom score to your AC. Not worth going Hungry Ghost, because the ki-drain only comes at level 5. Just a one, or at max 2-level dip. Probably after you have 1 or 3 levels of synth summoner.

Yeah I cannot find anything that is about punching people that isn't just a Fighter or Barbarian with a 2-hander sword that is any good. There's some rogue stuff that is nice, there's an alchemist archetype that gets sneak attack and would make a murderously effective hand to hand combatant with the mutagen and the buffs. But nothing based around jumping on things and punching people except synth summoner. Yeah. That's my advice.

Synthesist Summoner 20.

Optional; Monk 1.

Optional; Evangelist.

If unconcerned with enhancement bonuses and only needing the weapon special abilities, the 1st-level artificer infusions personal weapon augmentation [trans] (Eberron Campaign Setting 117) et al. and personal natural weapon augmentation [trans] (Races of Eberron 188) et al. grant weapon special abilities to one of the creature's weapons or natural weapons.¹ A wand of personal weapon augmentation (1st-level spell at caster level 1) (15 gp/charge) is inexpensive enough that even if adding the magic weapon special ability brash (MIC 30) to one's unarmed strike every encounter, that will still be cheaper than having such as permanent magic weapon special ability using the methods below. The magic weapon special ability furious (OA 125) is in a similar but worse position, as such an effect can only be granted by a wand of weapon augmentation [trans] (ECS 117) (4th-level spell at caster level 7) (420 gp/charge), likely making long-term use too expensive to use such an item to consistently have, for example, furious fists.

If the enhancement bonus is desired, an alternative is finding a way to cast the 4th-level Drd spell superior magic fang [trans] (SpC 136). This is a very expensive wand (a wand of a 4th-level spell at caster level 20 costs 1,200/charge), but that 60,000 gp item grants all of the creature's natural attacks a +5 enhancement bonus and, by the time such a wand can be afforded, the character probably won't be participating in 50 more fights during his career.² Note that this also assumes the spell superior magic fang can affect unarmed strikes. (While the spell magic fang implies that spell can, the spell superior magic fang lacks such language.)

However, all these all run afoul of the opportunity cost needed to activate them, so I understand why you'd avoid them. Attempting to generate such effects faster makes the price higher and makes the options below even more appealing.

Let's do some math!

Amulet of Mighty Fists (DMG 246)
The amulet of mighty fists specifically improves both the wearer's unarmed strikes and the wearer's natural attacks, but only grants both enhancement bonuses on attack rolls and damage rolls.

Amulet of Mighty Fists
   Enhancement Bonus         Cost
          +1             6,000 gp
          +2            24,000 gp
          +3            54,000 gp
          +4            96,000 gp
          +5           150,000 gp

Unfortunately, the amulet of mighty fists apparently can't grant the wearer's unarmed strikes and natural weapons magic weapon special abilities (that is, at least it can't without DM permission).

Bracers of Striking (Magic of Faerûn 155-6)
These cost 1,310 gp, weigh 1 lb., occupy the bracers slot, and grant the wearer the ability to make unarmed strikes as if the wearer possessed the feat Improved Unarmed Strike (very carefully not granting the actual feat). They can be made magical as if they were a blunt double weapon (hence any magical enhancement bonus or special ability costs double, the text implying that each "fist" (or head, I guess?) can't be magicked separately).

Bracers of Striking        Cost
 Enhancement Bonus    (1,310 gp)+
         +1            4,000 gp
         +2           16,000 gp
         +3           36,000 gp
         +4           64,000 gp
         +5          100,000 gp
         +6          144,000 gp
         +7          196,000 gp
         +8          256,000 gp
         +9          324,000 gp
        +10          400,000 gp

The item is not as explicit as it could be (the bracers can be modified with additional magic weapon special abilities but don't say those affect the wearer's unarmed strike, and I've read at least one argument wherein it was said seriously that the bracers' themselves must be used to beat a fool to employ their weapon special abilities), but if one assumes the enhancement bonuses apply to unarmed strikes and one's only making unarmed strikes, the bracers are a better deal than the amulet of mighty fists.

Necklace of Natural Weapons (Savage Species 58)
This item affects but a lone natural weapon unless more cash is paid. It's acknowledge that unarmed strikes are not natural weapons (but there's a counterargument couched in the fanged ring, below), so this probably doesn't do you any good, but it's included for completeness because you asked. The cost is multiplied by the number of natural weapons the amulet affects.

Necklace of Natural Weapons  Cost to affects natural weapons numbering...
      Enhancement Bonus        One    Two    Three    Four    Five
             +1                  2      4       6        8      10
             +2                  8     16      24       32      40
             +3                 18     36      54       72      90
             +4                 32     64      96      128     160
             +5                 50    100     150      200     250
             +6                 72    144     216      288     360
             +7                 98    196     294      392     490
             +8                128    256     384      512     640
             +9                162    324     486      648     810
            +10                200    400     600      800   1,000

Costs are in 1,000s of gp. Also add to the cost
+(600 gp × the number of natural weapon affected).

This makes the necklace of natural weapons a better deal than the amulet of mighty fists only if the wearer has but one or two natural weapons, except that the necklace explicitly can have magic weapon special abilities, and the DM must approve the idea that magic weapon special abilities can be added via the amulet.

Other Options
Below are a few options unmentioned by your question.

• The fanged ring (Dragon Magic 101) (10,000 gp; 0 lbs.) doesn't say it can be further magicked to grant unarmed strikes enhancement bonuses or weapon special abilities, but a generous DM may allow it. Further, the fanged ring implies that an unarmed strike is a natural attack, so it's useful for attempting to convince the DM that a necklace of natural weapons really can affect one's unarmed strike.
• The ward cestus (Arms and Equipment Guide 6, 10) (10 gp; 4 lbs), an exotic weapon, says that attacking with it "is considered an unarmed attack," and attacks with it deal the same damage as the wielder's unarmed strike. If the DM permits the alternative class feature city brawler to fight with a ward cestus yet still be considered unarmed, this is a viable alternative.
• Just in case you later become concerned with enhancement bonuses, a Drd20 charges 600 gp to cast the 3rd-level spell greater magic fang [trans] (PH 250) on an unarmed strike (possible according to the 1st-level Drd spell magic fang [trans] (PH 250)). A Sor20 charges 8,500 gp to cast the 5th-level Sor/Wiz spell permanency [univ] (PH 259-60) on that greater magic fang spell. Although vulnerable to dispel magic effects, this combination is inexpensive enough to get done several times over the course of an adventurer's career before acquiring +5 enhancement in some other fashion is more prudent. (Costs are according to Spellcasting and Services (PH 129), but actually locating such powerful casters is campaign-dependent.)

"So what do I buy?"

If you're using only unarmed strikes, you'll need the bracers. Fortunately, some of the weapon special abilities you want only require the weapon to be wielded, so, for example, you could get the following:

• brash (MIC 30) (+1 bonus; 0 lbs.) furious (OA 125) (+2 bonus; 0 lbs.) fury (HB 130) (+1 bonus; 0 lbs.) bracers of striking (Mag 155-6) (65,310 gp; 1 lbs.). Note: The bracers' description says that they "may be modified with special weapon abilities as if they were a blunt weapon." Ask the DM if that means the bracers need a +1 magical enhancement bonus before they can have further magic weapon special abilities.
• +1 brash (MIC 30) (+1 bonus; 0 lbs.) furious (OA 125) (+2 bonus; 0 lbs.) fury (HB 130) (+1 bonus; 0 lbs.) ward cestus (AE 6, 10) (50,310 gp; 4 lbs.). Note: As an exotic weapon, the cash saved from buying a ward cestus will probably spent overcoming the −4 penalty for nonproficiency.

And if, for example, you bind the soulmeld girallon arms (Magic of Incarnum 68) to your totem chakra and want magic weapon special abilities for that soulmeld's four claw attacks, you'll need the necklace.

• fury (HB 130) (+1 bonus; 0 lbs.) necklace of four natural weapons (SS 58) (10,400 gp; 0 lbs.). Note: See the bracers Note, above, the necklace description containing similar vague language.

Note: The magic weapon special ability fury (HB 130) (+1 bonus; 0 lbs.) needs to apply to every natural weapon and unarmed strike for it to be worthwhile, yet the special ability deals but an extra +1d6 points of damage (albeit untyped) each and only while raging, so, really, I suggest just skipping that magic weapon special ability. The same and more goes for the weapon special ability berserker (MIC 29) (+1 bonus), and more being that the weapon special ability is limited to two-handed weapons (which, with an inappropriately large magical ward cestus, is a possibility but a silly and impractical one).

Other Useful Items

• The beast claws (SS 49) (9,610 gp; 1 lbs.) are a pair of +1 spiked gauntlets (PH 117, 118) that grant the wearer 2 claw attacks with a +1 enhancement bonus and, if the wearer already has claws, the enhancement bonus increases to +2 and the wearer's claws deal an extra +1d6 points of damage. Note: Combined with the soulmeld girallon arms, these are pretty spiffy, but, given the item's age (and provenance!), the DM will likely update the description for a contemporary campaign. However, any DM that allows these into the campaign may also allow them to be further magicked.
• The collar of venom (BV 114) (50,000 gp; 3 lbs.) causes the wearer's natural attacks to become poisonous (Fort DC 14, primary damage 1d10 Con, secondary damage 1d10 Con).
• The demonhair shirt (Dragon #356 69) (26,000 gp; 3 lbs.) grants the wearer immunity to fear, and the wearer can spend 2 rages simultaneously to enter a demonic fury that, among other effects, grants the wearer impressive bonuses that explicitly stack with rage and frenzy (!) and makes all the wearer's weapons (including natural weapons) chaotic for overcoming aligned DR.
• The gauntlets of ghost fighting (MIC 216) (4,000 gp; 1 lbs.), among other effects, causes the wearer's melee attacks to affect incorporeal creatures normally.
• The ghoul gauntlets (MIC 104-5) (10,000 gp; 1 lbs.) grant the wearer, before making an attack, the ability, once per round, to declare one natural or unarmed attack as capable of paralyzing a hit foe (Fort DC 13 negates).
• The ring of adamantine touch (MIC 121) (6,000 gp; 0 lbs.) causes the wearers weapons (including natural weapons) to overcome DR X/adamantine as if they were adamantine. Given its rarity, this is likely a late-game purchase if at all.
• The tentacle extension (illithid) (Und 75) (32,000 gp; 2 lbs.) grant a lone tentacle a +2 enhancement bonus on attack and damage rolls, a 1-step size increase to damage, and an extra 5 ft. of reach. Note: The soulmeld displacer mantle (Magic of Incarnum 64) bound to your totem chakra grants 2 tentacle attacks. However, that doesn't make this a good deal.
• The wyrmfang amulet (MIC 148) (1,350 gp; 0 lbs.) causes the wearer's natural attacks and unarmed strikes to overcome DR X/magic as if they were magic weapons. Given how frequently this arises, this is likely an early-game purchase, later sold when better options become available.

¹ Fortunately, the infusion personal weapon augmentation targets but a weapon and its description only mandates it affect a weapon, saying nothing about the weapon being manufactured, so affecting an unarmed strike should be a thing even if not a monk.
² This overstates things a little. The character'll probably need to buy two or even three eventually. Each wand contains sufficient charges to use 1 charge per encounter for about 3 levels worth of encounters. A third wand tips things in favor of the amulet over the course of the character's career.