I know you've set your mind on gbb, and they're ridiculous fun, the WE ak is just giggle inducing to shoot. The guys have given good info... I will just go the other way a bit withe AEGs.
they are not hard to work on. Really. There are a few makes out there that are harder to work on, but there really isn't that much involved, even though a lot of people say there is.
IF you are scratch building a frankengun, yes, it will be hard to get proper airseal, and addressing airseal issues tend to be the most difficult.
The rest is not that hard, 3 gears a motor and 2 or 3 springs make up the mechanics of it.If you start with a gun that already has a good hopup/gearbox relationship, really the only thing you need to do is shim the gears, or, game it until something breaks THEN open it and address the issues, which when it breaks it's usually a piston that has lost the teeth, or a gear that's lost teeth.