i have an M120 spring in my M4. and thats all, nothing else is upgraded. I use 9.6volt battery, and i have broken 2 gearboxes. i would recomend an 8.4 unless you have a heavily upgraded gearbox (gears, piston set, bushings, air seal, head, the works). the casing itself should be reinforced too.
your best bet is a high MAH 8.4v