Ummm.... VFC's are totally gameable out of the box. Just plug and play. The only real semi consistent problem I've seen with them is pistons sometimes shredding (probably just a lemon batch?). Otherwise they're solid.
Otherwise like Colbalt Caliber said King Arms M4. The standard M4 version can be had for the low low price of $265 + shipping after that you can slowly put the external furniture that you want onto it such as drop in rail system (ie. KAC drop in SOPMOD rail, KAC BUIS, Tango Down Vert Grip, etc.).
The only consistent problem I've seen with King Arms is the Piston O-Ring being ever so slightly undersized sometimes resulting in air seal loss and low FPS (replace piston or if you have a plumbers o ring set lying around your house fit in a larger one). Also the shoddy cut off levers that can sometimes break resulting in a double or triple shot (or even full auto) when you're in semi mode.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE