Cyma CM.045 then.
Cloned from VFC so everything is held on via steel pins rather than self threading screws, comes with a 300mm-ish inner barrel that can be shortened to 247 without doing any sort of modification (24mm cw thread is actually a thread adapter and underneath there is a standard 14mm ccw), gearbox is your run of the mill 7mm Cyma V3 and while it's not a fancy retro arms box, it's still easy to work on provided you glue bushings in place.
As far as upgrade parts for it, there is almost as much choice as V2 boxes, a few parts are shared (gears, pistons, cylinders, cylinder heads, anti reversal latches), you can get some glorious hop units like Begadi Nebos, Combat Unions and Silent Industries.
Trigger is usually the weak point but nothing a Perun V3 + Clicker can't fix (you get adjustable pre-travel, over travel and trigger break force, something that is otherwise impossible on a v3 by design).
This for AEG, if you want a GBB then I have no clue, I heard GBB VFC rifles are good but I'm sure others here will give you more accurate information