It will take some work, but it's perfectly doable.
Enemies with special defenses
The players won't have access to see invisible, flight, magic weapons, or ghost touch. Therefore, if you include enemies that can fly, or have DR/magic, or are intangible, these will be huge challenges, and may be downright impossible. The easy solution is simply to not include such enemies, but the more interesting way is to treat them as nigh-invulnerable enemies that the heroes will have to figure out how to defeat. In a normal Pathfinder game, a ghost is a normal critter with a +2 CR template pasted on top, and requires some minimal preparation to defeat. In a low-magic Pathfinder game, a ghost is a mystery: why did they become a ghost? How can you persuade them to pass on, or at least let the party pass by peacefully? Can you persuade the local priest to perform an exorcism, and will it even work? Instead of "find monster, insert fireball," these types of encounters are now role-playing challenges, because they can't be solved any other way!
Alternatively, you can simply strip out the special defenses from enemies. Pathfinder assumes you have level-appropriate counters to special abilities anyway, so by removing those special defenses, you aren't going too far from the original intent. Adjusting the CR is left as an exercise for the GM, because it's going to take a fair amount of trial-and-error to determine what the right balance is.
Fixing armor class, and other numbers issues
Pathfinder assumes that both attack bonuses and defenses will be augmented by magic items. This partially balances out if the players don't have magic items, but consider giving everyone a +1 bonus to AC and all defenses every four levels. Don't make them pay a feat for it, just give it to them.
While you're at it, give your players bonus XP for the monsters they defeat, by calculating the XP as if the monsters were a higher CR. Since they're operating without magic, every encounter is going to be harder than what the DMG "expects" when it calculates XP per CR.
What will they do with their money?
Your players won't be able to buy gear that personally enhances their ability to make things dead faster, or grant them new solutions. If you keep to the normal loot rules, then the party will have far, far more money than they know what to do with. You have two options here: give them less money, or give them something to do with that money.
Let them invest in mercenary companies or land holdings. Let them become influential in the church, or their hometown, or even their country as their economic might and donations in the right places give them power that they would never be able to take with a sword. Favors in high places give characters some very powerful options.
Recovery after combat
This will require explicit house rules; you'll need to accelerate natural healing (heal a percentage of HP per day instead of a flat amount?), allow Heal checks to do much more than they normally do, grant the local clergy some extremely localized healing powers (they can heal people brought to their church, but not outside of their place of worship), and/or make this a political game rather than a hack-and-slash game.
If everyone's having fun, then it's a good game. It doesn't matter if the characters aren't optimized: as long as they feel like they're making a difference in the world and they're enjoying the game, then you're doing it right. The characters will be balanced, more or less: they all don't have access to magic, so intra-party balance isn't as much of a problem. You'll see that the players lack all of the magic-based solutions that you'd expect in a normal Pathfinder game, and you'll select the enemies more carefully, but things will work out fine. Let your players know that things will be a bit different than normal, and your players will go along with it; they requested this kind of game, after all.
Note: I'm not experienced at balancing homebrew content. I am simply comparing these items to official (DMG) material assuming it is balanced.
Item 1 is overpowered
Anti-magic field is an 8th level spell. an 8th level one-time use scroll according to DMG page 200 is a very rare item. So one that refreshes every day should be at least legendary, but even that might be too powerful.
Item 2 is inconclusive
there are several wands, such as wand of fireball that have up to 7 charges of 3rd level spells and recharge 1d7 of them at dawn, so that would suggest that 3rd level counterspell that has only 4 charges and recharges 1d4 of them would be underpowered. But that does not take into account action economy - to
use wand of fireball you have to use an action, but counterspell can use reaction. Given the fact that most of the time most characters don't get an option to use reaction that makes this item much more powerful. I haven't found a good analogue for this in official materials, so the value of 3rd level spell charges vs 'free' reactions is debatable.
Item 3 is at least rare
The closest analog to this is Mantle of spell resistance which is rare and states:
You have advantage on saving throws against spells while you wear this cloak.
First we have to evaluate the difference in benefit on all spell saving throws vs Dexterity saving throws. If we assume that spells are equally likely to use Dexterity, Wisdom, Constitution or one of the other stat saving throws that means this artifact is about 1/4rd as good as Mantle of spell resistance.
Second we have to evaluate the relative power of the effects. If the saving throw is likely to succeed 50% of the time, this item will reduce damage by average of 75% (50% on fail and 100% on success) while the Mantle of spell protection will only reduce damage by average of 12.5% (25% of time(if first roll fails but second succeeds) by 50%). That makes the effect of this item 6 times stronger than Mantle of spell protection.
As a result this seems to be about 50% stronger, but less versatile item than Mantle of spell resistance, so it should be at least as rare if not more so.
Item 4 likely underpowered
Currently it does not specify DC of the saving throw, that makes it impossible to tell how often this ability will come into effect, but generally if you are fighting in a party or with minions and the caster has more than one enemy in range, the usefulness of this ability would be quite limited, because choosing the second best target for a spell is usually almost as good as choosing the best target.
There might be some enemies that have only limited ranged spells, but even then the fact that the effect and on any aggressive action mean that the item is useful in only very niche circumstances.
Item 5 is probably ok
As you said there are no equivalents of this item anywhere in the official rules as there are no items that guarantee breaking concentration, especially not for a group of creatures.
But there is a spell Magic Missiles which makes the target roll 3 concentration checks without any chance to avoid them, and you can get it from uncommon item Wand of Magic Missiles with 7 charges 1d7 of them recharging each day.
Changing 3 concentration rolls to guaranteed failure, removing damage, but giving area of effect (which might not be that useful unless casters are plentiful) while increasing rarity seems quite fair.
Brooch of shielding is balanced
My method of evaluation is comparing items to official content. As Brooch of shielding is official all I can say is that it is balanced
Item 7 gets better with tougher foes
Evaluating this item is tough because while it seems very similar to armor of resistance and magic weapons seem to be about as frequent as some of the damage types the problem is that magic weapons are mostly possessed by higher level opponents unlike damage types which are distributed more randomly or based on theme. That means that this kind of armor would be next to useless for one facing low level mundane enemies such as brigands and orcs, while being extremely useful in battles against fiends and high level adventurers (unless they have backup non-magical weapons).
Item 8 seems close to balanced
Looking at Legendary items that provide protection against spells there are Ring of spell turning, Robe of Archmagi and Scarab of protection - all of which give advantage to saving throws, but also have additional effects. While resistance is clearly superior when targeted by spells that are used primarily for dealing damage, most of them can also be partially mitigated by succeeding on saving throw while resistance does nothing against crippling debuff spells.
In the end resistance vs advantage on saving throws seems quite close, so this item seems quite balanced.
I'd like to build a magic system
in which every spell is a one-off.
Learn and burn, got it. A way to control this aspect is with the cost of learning, and/or the cost of burning (involving actually destroying the information once it has been used)
"I'm thinking that the time and place of the casting would be important in how the spell was constructed and so a mage couldn't simply trot out the same ritual and get the same effect each time, because this would probably lead to players 'inventing' the fireball spell and then just using it at every opportunity."
The Sun, Moon, and Stars are a great way to alter what effects come into play. You could literally use the night-sky as your proverbial dice and base the outcomes on the time of night and the position of constellations, real or imagined. It would literally take them eons to get the same result, and as such, would thusly require different spells or spell components (physical or otherwise) to acheive the same spell they completed hours, if not minutes ago. If you'd like something a bit more observable, you can use the real world time to affect the different aspects of the spell effect, whether that ice ball becomes a fire ball, or that mind control becomes charm or berserk effect. The balance in this case would come from the position of the stars being favorable or unfavorable.
How do you apply constrains to such a system such that more powerful characters can get more powerful results while not specifically defining what effects are available?
On one end of this equation when you first create spell effects, like in Shadowrun's latest edition, you have to acquire rather expensive spell components.
A more powerful magic user ("Adept", "Magician") can afford those more expensive spell components by virtue of their previous success (power).
On the other end of this equation is the "repair" cost for casting. More powerful magicians could end up paying a smaller cost for the same effect of a less powerful magician, or have the option of paying more of a resource to have a bigger effect.
A friend of mine created a "Nomenclature Casting System" involving "true names" of people, objects, ideas, locations, and the like. Each true word had a value. Bigger constructions of sentences had a bigger value and thusly did a larger effect. You could try something similar in requiring a "true name" for each spell that is consumed by the caster for that particular target. (The Balance in this case comes from being price prohibitive.)
If that won't work for you, perhaps I can suggest randomly generating each spell from a list each time it is cast. Let the magician choose the element, roll for the different aspects of the spell, let them see what it can/will do and let them pick how to attach it to the world at large. In this instance, most things are available, and are once again dependent on your general ability with magic. As the character's power grows, let them add more aspects to the list and slough off the used ones into the trash bin. The Balance in this case comes from what you are willing to allow the casters to use as a list of effects.
This style of magic changes with each casting and should keep things rather new, pending that you are able to continue adding words to the list and keep track of ones that are used already. Words that are lower on the list could have more power, or that could also be randomized within a rage appropriate for the caster.
And if THAT doesn't work for you (too "crunchy", perhaps?) you could have the players come up with an adjective to describe the action. The better the adjective, the better the bonus. It's incredibly ill-defined, limited to what words you accept, and even nebulous in what sort of benefit it gives the character. (And it makes you break out the Thesaurus!) Once a player has used that word, it can't be used again, or can't be used until some sort of time limit expires. In this idea, everything is available, and power is only limited to the bonuses you give for creative word usage. (It has limited use in a continuing campaign, but breaking out dictionaries and thesauruses will make everyone at the table look like they are reading from tomes. Added bonus!) The balance in this case comes from the completeness of the source they use, as well as a consensus between the other party members and the GM him/herself.
If you're looking for something more punishment based, you could borrow from Changeling the Dreaming (White Wolf Publishing) and lower one of the good stats/raise a bad stat ("Banality") of a character for repeating the same action in the same manner over and over again. This allows some repetition, but has the chance to seriously damage the character.
Actually,... I'm going to go ahead and more fully recommend you use Changeling the Dreaming's Magic system and just rename Banality to something more appropriate for your campaign. You get a sphere of influence and the ability to affect a certain type of target. Call "No repeat-sies: if you do, it fizzles," and watch them turn trees into houses and flip people like quarters. The system is already put together. I have had first hand experience with it, and while a few things may be confusing at first, the system gets easier to use over time. "C:tL" uses "Wyrd" as a "mana pool". Out of Wyrd? Out of Mana. You can opt out of spending mana in the first place by following a particular Ban, negating its cost. The Balance in this case comes from difficulty of skipping costs (For instance, Eat naturally formed ice before wreathing yourself in an aura of flame -- Great for Summer/Desert campaigns).
So, in summation, you could:
make it dependent on the motions of the heavens (month/day/year/astrology) or the hands of the clock.
make it a randomized list within a range of possibilites and scratch off the ones used.
make the characters describe it and never use that description again.
Hack away at Changeling: the Dreaming for its magic system and rename things that don't make sense.