Im gonna recommend either a prom or madbull barrel (6.03 is my diameter of choice) (evike sells pre rhopped madbull Barrels for $65 usd) a modify flat hop bucking (hard or soft depending on fps) modify nub (if u buy the hard bucking,soft comes with a nub) r hop and prowin g36 hop up. If you want 400+ fps you'll need a semi only mod (max fps will depend on your countries/fields fps limits)
I do not know anything for the gearbox but I would get a mosfet, deans, and either a 11.1 or 7.4 Lipo
I'm also gonna recommend a peq box due to the battery compartment being a pain to access