Custom armor items - that should be very easy to do. Right now we are creating items manually, so I cannot describe how you will go about it in the final game, but making armor with different magical effects or stats bonus will be easy. This is my personal opinion, based on what I know about NWScript. But these are problems that are common to learning any of the above languages. It does accept 5.0.
Modules are a framework to contain one complete adventure.
They are nothing more. So running a number of smaller modules on the same system vs running one large module will be quite similar, only the single module will have less OS overhead. Skinning is still a bit up in the air. We have no final implementation for it, but we want to include the choice to use custom skins. Right now we need to get all the basic functionality in the game before we start tackling the optional features.