You cannot wear armor that is not sized for you
Most items are allowed to work when missized, and weapons work with a penalty, but no such rule exists for armor, and armor is referred to as having a particular, fixed size.
Armor spikes don’t necessarily have to be the same size as the armor they’re on
There simply isn’t a rule anywhere that says so.
At first glance, this seems absurd and an oversight, but I think a case can be made for “extra large” spikes. This is most notable in the case of armor made for half-giants and goliaths, as their Powerful Build means they are somewhere halfway between Medium and Large, and can use Large weapons without penalty though they wear Medium armor. For a goliath, it makes sense that they would wear Medium armor with Large armor spikes.
The handedness of armor spikes is undefined
Armor spikes are supposed to be Light weapons, do not get used in any hands, and at no point do the rules address over-large spikes directly. In theory, Large armor spikes on a Medium creature would now be one-handed weapons and thus eligible for wielding “in two hands,” but there is no way to actually use two hands while wielding them. Thus, I do not think it is possible to get the 1½ Str to damage or the 2:1 Power Attack ratio with this method. They do get the Str to damage and 1:1 ratio of a one-handed weapon, because that is what they are. The rules do not actually say that one-handed weapons get those while “wielded in one hand,” unlike the two-handed versions.
But you could take things a step further: Huge armor spikes on a Medium creature are two-handed weapons. As with one-handed weapons in one hand, two-handed weapons don’t actually specify that you only get the improved damage when wielding them in two hands, because that’s assumed. The text is “two-handed weapons and one-handed weapons wielded in two hands.” So in this case you would get improved Str to damage and Power Attack. And a −8 penalty to attack for wielding a ludicrously oversized weapon.
The armor still does not take up any hands, in any of these cases.
This really does not seem like the right way to go
You have a monster that’s given a higher CR than it probably deserves, against PCs nearly twice that CR in the first place. The sensible thing to do here would be to houserule them into something better, or at the least give them class levels (I recommend Fighter 6 with the Dungeoncrasher ACF from Dungeonscape, or else Crusader or Warblade). This particular collision of the rules is wonky, and still will not provide particularly effective combatants.
Counterintuitively,
Avoid Weapon Special Abilities that Inflict Damage
If your PC has been taking feats like Favored in House (ECS 53), and Least Dragonmark (mark of finding (ECS 64)) (ECS 53), and Skill Focus (Bluff) (PH 100) so he can better make Seduce to Learn Secret checks while the other PCs have been specializing in monster murdering, and they realize you aren't contributing much in a fight, explain that you'll need their help. As a swashbuckler (who doesn't cast spells) you were going to need it anyway. That's not an insult but a fact of mid- and high-level play.
You'll need their help because the first weapon special ability you add to your rapier is spell storing (DMG 225) (+1 bonus). No other weapon special ability scales for damage as well as it does because it scales with your party, one of whom should be happy to refill your +1 spell storing rapier free of charge at the end of a slow adventuring day. If you must fill your own tank, take ranks in the Use Magic Device skill and load your +1 spell storing rapier using a wand of combust [evoc] (SpC 50) (2nd-level spell at caster level 10) (300 gp/charge) or a wand of vampiric touch [necro] (PH 298) (3rd-level spell at caster level 20) (900 gp/charge) or a wand of another spell that the Comments will inevitably suggest.
It sounds like you're in a game wherein sources of magic items are restricted. If you aren't, also consider the weapon special ability bloodstone (MIC 29) (+1 bonus), which is like the weapon special ability spell storing that only holds the spell vampiric touch but the touch is modified by the feat Empower Spell (PH 93) for free. Seriously. Then add the weapon special ability power storing (MIC 40) (+1 bonus) if there's a psion available; my unfamiliarity with psionics means I don't know what's best, but there must be something awesome and Eberron is psionics-friendly. Then virtually guarantee your attack hits by adding the weapon special ability impaling (MIC 37) (+1 bonus).
If you are confined to the Dungeon Master's Guide for your weapon, the only other serious choices for damage are the weapon special abilities bane (DMG 224) (+1 bonus), picking whatever the DM's throwing at you a lot, and holy (DMG 225) (+2 bonus) or, y'know, unholy (DMG 226) (+2 bonus) if you're an evil swashbuckler: a lot of your foes will be evil (or good) anyway; punish them for it. If chasing just damage buy the weapon special abilities magebane (CAr 143-4) (+1 bonus) and vampiric (MIC 45) (+2 bonus) if allowed.
The flat +5 damage from the weapon special ability collision (MIC 31) (+2 bonus) is the best you're going to get, but also consider, if it's permitted, the weapon special ability fierce (AE 96) (+2 bonus) that lets the weapon's wielder to take a penalty to his Armor Class up to his Dexterity bonus to add the the penalty's absolute value as a bonus to the weapon's damage.
Although others may disagree, most of the other damaging effects from the DMG and elsewhere inflict energy damage that a high level creature's energy resistance will easily absorb, employ effects high level foes are often immune to (e.g. ability damage and ability drain, critical hits, negative energy effects), or have saving throw DCs that are too low to be viable.
Effects That Don't Inflict Damage, But Take a Look
The weapon special ability dispelling (MIC 33) (+1 bonus) et. al. will make you popular with your group even if you aren't inflicting a lot of damage. The same goes for the weapon special ability revealing (MIC 42) (+1 bonus), which eliminates miss chances for your party; in high-level play miss chances are commonplace, and everyone will thank you even if you hit only once.
The elemental power weapon special abilities from DMG2 254-5 et. al. range from a +1 bonus to a +4 bonus and permit summoning of an elemental of increasing size based on the bonus purchased. Being an elemental-summoning swashbuckler may sound strange, but the value of minions with cool powers can't be overstated--ask any necromancer.
I don't like abilities that allow saving throws nor do I like abilities relying on critical hits, but the weapon special ability prismatic burst (MIC 40) (30,000 gp) is an exception to both. The flat cost means it's possible to add it to a weapon that's already full of other bonuses. The weapon special ability sending (LE 152) (+4 bonus) is also valuable, despite only activating on a natural 20 and a successful critical roll; a no-saving-throw teleport effect versus a high-level foe is more likely to remove him from battle than almost any other effect, although by the time this is avaialable most foes should have a way of returning to the battlefield shortly.
I've not heard of a DM actually allowing the weapon special ability smoking (LD 180) (+1 bonus), but I suggest it because it exists. It's a defensive ability, but a spectactular one, and defensive abilities keep you alive long enough to use offensive abilities. The weapon special ability spellblade (PG 120) (6,000 gp) is just as cool; pick a targeted spell the DM loves, ignore it, and even huck it back at the caster.
The weapon special ability Keeper's fang (ECS 266) (+4 bonus) renders creatures you kill with the weapon unable to return from the dead via the spells "resurrection, raise dead, reincarnate, or similar effects"; the weapon becomes evil and makes you look like a jerk to some folks, but this is such a powerful ability in any typical Dungeons and Dragons campaign that its utility can't be ignored.
Don't Neglect Accessories!
A broken weapon made from aurorum (BE 38) (+4,000 gp) can be made whole as full-round action, but a weapon made from riverine (Sto 128) (+2,000 gp/lb.) is just immune to damage.
A weapon made from morghuth-iron (AE 14) (+4,000 gp) is constantly naturally poisonous, but the poison's saving throw DC is low, the weapon's wielder takes a -1 penalty to attack rolls and damage rolls with the weapon, and many high-level creatures are straight-up immune to poison. This is, however, far cheaper than many effects, and there's no use limit.
A weapon with the template pitspawned (DMG2 278) (+1,000 gp; 0 lbs.) grants the wielder a +2 bonus to critical rolls.
A wand chamber (Du 34) (100 gp; 0 lbs.) is a space in the weapon for a wand which can be used without drawing the wand; perfect if you've ranks in the Use Magic Device skill. A hilt hollow (Du 33) (300 gp; 0 lbs.) is a space in the weapon for a potion which must be extracted but it's easier than normal. Get both. An oil chamber (Du 33-4) (1,000 gp; 0 lbs.) is expensive, but can hold an oil of greater magic weapon [trans] (PH 251-2) (3rd-level spell at caster level 20) (DMG 230) (3,000 gp; 0.1 lbs.) or ghostoil (AE 34) (50 gp; 1 lb.). A triple weapon capsule retainer (CAd 121-2) (450 gp; 0 lbs.) holds 3 alchemical weapon capsules (CAd 122), which do everything from more damage to silvering your weapon on the fly.
You'll want a weapon with at least a +3 enhancement bonus (not just a +1 weapon with at least 1 additional +2 bonus or 2 additional +1 bonus weapon special abilities!) if planning to attach the most powerful augment crystals (MIC 221). Start with the crystal of return (least) (MIC 65) (300 gp; 0 lbs.) now if you've not the feat Quick Draw (PH 98) or another way to draw a weapon as a free action. Purchase more augment crystals as needed for the campaign.
Suggestions
All bets are off if the campaign allows more sources than these.
- A +1 bane (humans) holy spell-storing adamantine rapier (53,320 gp; 3 lbs.) if you can only use the Dungeon Master's Guide.
- A +1 bloodstone collision dispelling impaling prismatic burst spell-storing vampiric adamantine rapier (195,320 gp; 3 lbs.) if you can use the Dungeon Master's Guide and the Magic Items Compendium. Perspective: This weapon costs nearly two tons of gold.
Best Answer
I cannot hope even to come close to the "Tome of Necromancy" by Frank and K for advice--it should be mandatory reading for any necromancer--, but here're some starting points for manufactured undead.
Creatures Available Using Animate Dead
The 4th-level Sor/Wiz spell animate dead [necro] (PH 198-9) lacks the versatility of the 6th-level Sor/Wiz spell create undead [necro] (PH 215) and the 8th-level Sor/Wiz spell create greater undead [necro] (PH 215), but Wizards of the Coast didn't ignore it completely.1 In addition to the skeleton (MM 225-7) and skeletal dragon (Dr 192-3) and the zombie (MM 265-7) and zombie dragon (Dr 197-8), the following undead can be created via the spell animate dead by meeting additional requirements and providing the additional components beyond onyx worth 50 gp per HD.
The Necrosis Carnex Meets Your Requirements
While it would be beautiful to create bone nagas, unless the DM's willing to handwave the fluff (and, really, the DM shouldn't--bone nagas are ridiculous and the fluff required to learn and perform the ritual is pretty cool), the necrosis carnex is the winner. Although it only has an Intelligence score of 3 and can't speak (it only "understands [its] creator's orders"), it can nonetheless lead by example and even gain class levels.
Have the DM determine if necrosis carnexes "make few attempts to communicate" (MM4 105) because they're--I don't know--just jerks or something, or if it's because they physically can't communicate. If the former, necrosis carnexes don't need the feat Surrogate Spellcasting (SS 39-40) to become casters, but, if the latter, they're out of luck without modifying all their spells with the feats Silent Spell (PH 100) and Still Spell (PH 101). Even lacking class levels, the smallest army of manufactured undead should contain at least one necrosis carnex anyway; its supernatural ability necrotic touch makes it an undead army's medic (This can be superseded by quantities of the hazard black sand (Sa 20), which can be created temporarily via the 3rd-level Clr spell black sand [necro] (Sa 111)).
"Can Hit Dice Be Added to Undead?"
A creator can't add Hit Dice manufactured undead as part of their creation (q.v. here). Manufactured undead have a set amount of Hit Dice and must gain Hit Dice via either acquiring class levels, a special ability, or fluff text (e.g. undead who exist long enough sometimes acquire the template evolved undead (LM 99-100)). However, a variety of means exist to upgrade manufactured undead as part of their creation.
Methods of Upgrading Undead During Their Creation
Too many base and prestige classes to list grant bonuses to manufactured undead, but of particular interest--and available to any creature with who takes even a single level of wizard--is the necromancer specialist wizard variant class feature enhance undead, an extraordinary ability, which says that
Emphasis mine. As the creature is still a necromancer after taking the level of wizard no matter what other classes the creature possesses, undead manufactured via clerical spells get this same bonus.
Barring shenanigans, one character can't possess all the feats that improve manufactured undead. There are just too many (even to detail, which is why I only sourced them). Further, too many such feats grant inconsequential bonuses that become consequential only all at once in a big pile. Instead of permanently taking these feats, if possible, before manufacturing undead, acquire them via the power psychic reformation [telepathy] (XPH 127) and, when finished manufacturing undead, use psychic reformation a second time to return to the original build. Sure, it's dodgy, but the game is cruel to make folks think that a +2 bonus to manufactured undead's natural armor via the feat Hardened Flesh is worth a 1/7 or 1/8 a character's total feats for his entire career.
Below is additional information that may help the character create a more formidable undead army.
The Spellstitched Template
As soon as a creature who wants to master undead is able, he should apply the template spellstitched (CAr 161-2) to an undead he controls--preferably to an undead he can keep in a safe place and somehow grant an obscene Wisdom score. If the undead's Wisdom score can be sufficiently increased while the template spellstitched determines what spell-like abilities the spellstitched creature can employ, just one spellstitched undead can make (un)life as a necromancer much more interesting, saving him mountains of gold and bottles of XP.
"I Need More Undead!"
Is there such thing as too many undead minions? Of course not. The holy site sepulcher of Wee Jas (CC 156-7), the armor and shield special ability undead controlling (DMG 219) (49,000 gp; 0 lbs.), and the magic items night caller (LM 79) (7,000 gp; 0 lbs.) (and whether this supersedes Azun-gund (SC 32) (3,000 gp; 0 lbs.) is the DM's call), rod of undead mastery (MIC 175-6) (10,000 gp; 3 lbs.), porcelain mask (OA 141) (27,000 gp; 2 lbs.), mace of unlife (AE 112) (138,312 gp; 8 lbs.), rod of restless death (ELH 139-40) (625,000 gp; 5 lbs.), and a host of other items allow a creature to control more undead than normally allowed.
The feat Planar Touchstone (PlH 41-2) while linked to the touchstone site Catalogues of Enlightenment (PlH 166-7) can give the character the granted power of 1 of the Domains deathbound (SpC 272-3), deathless (ECS 105), or necromancer (ECS 107), any of which will improve the creature's ability manufacture or control undead.