[Hectic]

Is it for an indoor primary or an outdoor sidearm?
If It's for indoors go TM, run it on duster and do noting to it besides maybe an inner barrel and some sights.
If it's for outdoor go we or hk3p (or whatever the he'll they are called) run the we on propane or the hk on propane or co2. Put an inner and a bit of hop up work (maple leaf wheel and nub thing and a rubber) and away you go. Avoid auto glock unless you don't mind the potential for failure (more parts to break, little weaker in the bbu area do to the fire selector and such)
Either option you can start putting nice slides and upgraded recoil and hammer springs and such but you don't really .need those until something becomes a problem.