Frankly playing Kingmaker for Pathfinder in a low magic environment is difficult... Not impossible but difficult... But it's as difficult as to make it suitable for another game entirely...
In all case you have to redo almost all encounters from the ground...
I'm not familiar with Iron Heroes but if it changes the heroes it changes the ennemy... So it maybe a way to simplify a bit the changes you've got to made but you still have to remade them..
First when I read your world description my first incentive was to say : "Go Warhammer" (1st Ed. is my pref but from each their own) the whole world you describe IS the Warhammer background :p
Now there's a lot of Fey stuff in Kingmaker, I won't go in detail but there's fey in those woods and fey are an important part of the AP, be it for fun stuff or some serious stuff... ;)
In a settings without fey you will have a hard time with Kingmaker... Once again not impossiblme but a whole lot to rewrite...
If you like Kingmaker do you like the AP story or the Hewcrawler aspect ?
If it's only the Hexcrawler aspect look at those product from Frog Good Games they can help you built your own Hexcrawler game ;)
Edit : Forgot to say : the Ultimate Campaigns rules to make a Kingdom are awesome but you also want to take a look These books from Legendary Games that will give you much more options for your game... Using those rules for another game is not as difficult as it seem, you can use them almost as it is... ;)
I always been lowish-magic in the way I write and run things - although nowhere near as low magic as you are aiming for.
However, with PathFinder APs I found it easier to change my style rather than change the APs. Mind you, most of my players still think I am a very niggardly DM. You will have to rework an awful lot of material (book 3 onwards) as the concepts are quite magic-dependent, although it is god magic most of the time. They drop some incredibly powerful, artefact level, items - which technically are usable by PCs - although you probably won't want the PC to use them :) n a Low Magic world they would likely finish up as Evil Despots!
Books 1 and 2 should work well enough though - which gives you time to work on the rest of the material.
For me, running Kingmaker with restrictions on the amount and type of magic available to the PCs is working. That being said, I did not make the restrictions as strongly as you are aiming for in your campaign.
I have a little bit of familiarity with Iron Heroes but not sure if it is quite enough for the pcs to compensate for the lack of magic they are hampered by, whereas it seems like the opposition do have full access. As JohnB stated, you are probably fine with minimal game design changes to run the first two books, but thereafter it becomes increasingly difficult.
For example, the end boss in Book 3 is capable of sending powerful daemons against the heroes and those often are difficult to defeat without the aid of magic of some sort. The end boss of that book can permanently paralyze the heroes if they fail a save - how are the heroes supposed to reverse/remove the paralysis then?
A number of the random encounters and set encounters in Book 4 to Book 6 assumes that the heroes either have immediate access to the magic needed to overcome the encounter/difficulties or is capable of getting with little difficulty. Without the aid of magic, the encounters become significantly harder or simply impossible to overcome, simply because of what the opposition can do with magic.
Now, that is not to say that you cannot run Kingmaker with Pathfinder and minimal magic available to the heroes to use, but you will have a lot to change. I partially offset the magic handicaps against the heroes in my campaign by allowing them to "learn" martial manouevres (from 3.5 Tome of Battle), even if they are not actually any of the martial adept classes. A lot of those martial manouevres provide significant tactical advantages.
But the harder stuff for you to address will be the lack of divination that the heroes will need. I am not sure yet how we can fix that.
CB
Also, what about healing/restorations? Does Iron Heroes address those concerns?
Another thing is that for hex-exploration, are you going to keep track of supplies?
CB
There might be a way to compromise a bit, but not sure if this is where you want to go. Think of "magic" more as "technology" or "progress".
Some examples: Cure Wounds is a touch spell. What's to say that Cure Light Wounds isn't just the cleric dabbing some healing ointment from a nearby plant on the fighter's open wound? The fighter starts to feel better as it numbs the pain and starts to clot the blood, etc.
Your basic magic weapon confers a bonus to hit and damage (obviously not talking about other magical effects here). In the movie Kill Bill, everyone marveled over the Hanzo Swords even though they weren't magical, yet everyone knew that they were simply better. Priceless even. So a +1 Longsword is really just a long sword that was made by a really good weaponsmith. A +2 was made by an even better smith, etc.
Fey are weak against cold iron. What's to say that its not just treated as a powerful allergy? Sort of like how some people are really allergic to latex.
The wizard who casts a fireball could really just be throwing a molotov cocktail.
The Cleric who casts bless could simply be chanting a prayer that fills her comrades with confidence or unsettles her opponents.
Obviously this won't always work for everything, and things like DR/Good become particularly complicated, but I think it at least opens the door to possibilities. Heck, even an amulet of protection could be seen as a lucky charm and thus the wearer of the amulet effectively has better saves simply because she believes she does (think back to the Harry Potter books, IIRC there was a Quidditch game where Ron does particularly well because Hermione tells him she cast a spell on him that would make him great. She never did, but he believed she did so he went out there without fear. In essence, its a placebo effect.
Just a thought on how you could still convey the feel of the gritty setting while still getting around some of the bigger problem areas.
There might be a way to compromise a bit, but not sure if this is where you want to go. Think of "magic" more as "technology" or "progress".
Some examples: Cure Wounds is a touch spell. What's to say that Cure Light Wounds isn't just the cleric dabbing some healing ointment from a nearby plant on the fighter's open wound? The fighter starts to feel better as it numbs the pain and starts to clot the blood, etc.
Your basic magic weapon confers a bonus to hit and damage (obviously not talking about other magical effects here). In the movie Kill Bill, everyone marveled over the Hanzo Swords even though they weren't magical, yet everyone knew that they were simply better. Priceless even. So a +1 Longsword is really just a long sword that was made by a really good weaponsmith. A +2 was made by an even better smith, etc.
Funny that you mention this because one of the other things I did to help offset the lack of magical weapons above +3 in my Kingmaker campaign was the Master Crafter base class introduced in the 3.5 Dragonlance setting. They can create weapons that have a +5 Mastercraft bonus to attack (doesn't stack with enhancement bonuses) or improve one of the statistics of an armour (AC, AC penalty, etc.). Although such mastercraft items were pretty expensive, they were not as expensive as magical equivalent. Plus, I also used the Mastercraft qualities from a Dragon Magazine to add additional enhancements to weapons and armours.
As for the lack of spellcasting for the pcs (or at least the absence of 1st tier spellcasting), maybe something akin to Pokémon where the heroes can obtain spellcasting services from defeated monsters or as rewards for quests?
CB
Another thought that crossed my mind to help especially in the defenses department (saves, AC, etc.) is you could adopt the inherent bonus system that I believe was introduced for 4ed D&D as an alternative rule system. Essentially, at every X levels, the PCs get +1 to their saves, to hit, etc. Its not magic but instead replaces things like amulets of protection (same bonus). This simply reflects that a seasoned adventurer is harder to mind control, or has developed good instincts (reflexes), etc.
Yeah, I'd say you're better off with another system entirely. Pathfinder has a huge bunch of baked-in assumptions. Even basics, like the characters having the ability to magically heal wounds, or having magic weapons, are going to cause big trouble when it turns out monster wounds are incredibly lethal and anything with damage reduction/magic is going to largely invincible. And that's just the basics.
Switching systems - and accordingly trading out the monsters for versions which don't depend on magic to defeat - is an excellent start.
Outside of the mechanics, you'll run into several story problems with Kingmaker. Needing to murder all the fey/kobolds/centaurs because they're weird and inhuman will a) add a lot more combat to the adventure that is necessary, and b) make several story points invalid.
Take an encounter from Book 2, for example, which involves a fey creature arguing with lumberjacks over logging wood in the area. It's meant to represent the need for compromise, encourage the PCs to get involved and make decisions about how they manage the inhuman elements of their kingdom. If the answer is culturally "Kill it with fire", there's no interesting encounter here (because either the lumberjacks have already killed the fey or the PCs will do so momentarily). And there are several such encounters throughout the books.
Short answer - yes, it can be done, even with Pathfinder, but prepare for a LOT of work rebalancing the game and rewriting the story. Writing your own version and borrowing some inspiration from Kingmaker to work with a seperate system will be a lot easier.