Let me ask a question. If that is what you really want, why worry about where AC comes from at all? Why consider armor or magic in the equation at all? I mean, when you get down to it, the majority of the complexity you are describing in the situation that prompted this post is armor and magic (plus an 'unhittable' class feature very much like what you are describing). Why not just assign an AC fo all characters based on level and qualified by class and perhaps your ability score? Whether you put on armor or are nimble and fast is just color, regardless you end up with the same AC. If you level, your AC goes up and this is described merely by color - you found some magic armor or you got more nimble. Which it is is meaningless in the framework of the game, and the stated goal - ensuring simplicity of resolution and balance is really easy to achieve.
Likewise, why worry about what weapon the combatant is using to do damage? Why not just assign each character a die based on level and class, possibly modified by a single ability bonus, and consider the weapon to be just color? It doesn't really matter if the character is pummeling someone with fists or stabbing with a knife or slashing with a sword. Again simplicity of resolution and balance is really easy to achieve. What do you achieve by doing something different?
I have a similar question, but for a different reason. Why care at all how the character got his or her AC? The concern is that there are too many fiddly bits to keep track of to get that AC, but why does the DM need to track it? If the player is enjoying that aspect of the game, let the player enjoy it. The DM just needs to know the final number, which the player can provide.
@ RangerWicket - You say that you want a cinematic feel to the games where you don't have to hunt for obscure bonuses to challenge the PCs. Fine. If a character has an amazing AC and can't be hit, instead of thinking about it as a mechanical problem, think about it from the cinematic point of view. What would the enemies do if there was a character that deftly ducked, dodged, and weaved around their clumsy attacks? Well, obviously, they would begin to try other ways to beat him. Grappling. Tangle bags. Nets. Gang up and "aid another". Surround and flank. Et cetera, et cetera. As someone else said, you want this to be some sort of super spy mission, but you don't want to have super heroes. Sometimes grunts need to get creative, or they will be completely overwhelmed by the heroes. I don't see where the problem is with that, and in fact, I think it can lend itself to MORE cinematic play. Stop looking at it as a mechanical challenge, and instead look at it as a tactical challenge.
Also, I don't know if this applies to your situation, but if you are attempting to use classed NPCs to provide a challenge, keep in mind that the CR guidelines for NPCs are woefully inadequate. Level -2 or -3 seems to be a decent starting point for NPCs. You might even consider level * 3/4.