Not sure if it's still true or if there's something better but the Tanio Koba Designed KJW M4's (the V2's without the "rubber band problem") work right out of the box without any upgrades or anything like that necessary. Except for maybe the Cradle Velocity kit so you're not shooting hot in summer or if you're playing in CQB you can turn it down.
From what I understand if you buy a WE Open Bolt you'll need to immediately upgrade it with some reinforced RATech parts (unless you get one with it already installed) and you might as well put an NPAS kit on it while you're upgrading with reinforced parts.
I've never owned one but if I were to get a GBBR I think I'd get one with the WA Magna system, not piecemealed cheap crap thrown together but like a G&P WOC or ViperTech.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE