I used to be an airsoft mechanic, perhaps I can help. You're going to want an automatic electric gun (AEG), there aren't any good full-auto gas guns that aren't stupidly expensive last I checked, and the semi-auto ones are high maintenance, low capacity, and incredibly hard to modify (and the gas ain't cheap, either). The good things about electric airsoft guns is that you really only need 3 parts, and they can all be taken out separately and usually easily. A gearbox (which houses a motor and an air piston to propel the BBs, these are self-contained, and you can use one straight out of another gun), a magazine and feeding mechanism (also can be taken out of another gun), and a barrel (same deal). The main problem you're going to encounter is that the gearbox is the size and shape of a small pistol, and pretty much always has to be directly behind where the magazine feeds into the barrel for the magazine to load BBs successfully. Some are smaller than others, but all are based on a few templates. It's pretty self-explanatory, but if you'd like a more detailed guide, let me know.
