The marui M4s are outdated, they're only reliable when stock (at 280fps). Upgrading them to perform at the same level as, and have a metal body, will cost you more than just buying a new gun.
Classic army has never made GOOD guns, they've made a lot of crap and a lot of okay guns. The only reason any older player will THINK they were good is because way back 8-10 years ago your only choices in airsoft were a reliable $500 plastic marui or an unreliable $650 metal CA.
Their internals are crap, their externals are weak, their paint is too shiny, their 249 is inferior, they're just slightly off tm spec to make aftermarket gears whine constantly, and they've been upstaged by G&G on just about every gun.
G&P make excellent upgrade bases (I've got a love/hate relationship with G&P), but they're got some of the worst stock gears, unreliable pistons, explosive (aptly named) piston heads, and often times assembled incorrectly from factory. Their motors are also garbage. Really their only saving grace is that they have some of the toughest metal bodies and use RS threading on the receiver. Not sure if it's still true but the MK23's have semi-proprietary gearboxes that had the holes for the spur gear misaligned as well.
The make THE best 249 on the market, though.
Basically you'd be a fool to run a stock G&P lol
VFC is the best gun to keep stock and upgrade later.
G&G is a cheaper gun to keep stock and upgrade later, externals aren't quite as good. But this is the standard beginner gun.
KWA is actually pretty decent as a stock gun until the cylinder head inevitably explodes.
Some manufacturers will have better and worse specific rifles. Like G&G's AK5C is just a horrible gun, and marui make awesome sniper rifle bases.
BOLT and KRYTAC also make some pretty high end guns apparently, haven't worked on either myself but I know other gunsmiths who have.