G&P is probably one of, if not the best name in the industry. They're right up there next to VFC.
Usually, the G&P MOE is like $350, but there are always cheaper alternatives. (or deals for them here on the forums of course
)
It is true about their gears. the axle on the sector gear is almost always destined to snap. That being said, all you need to do is replace that gear with a better one right off the bat, or just get a new gearset entirely.
The piston is fairly good. It will not last you forever, but you wont need to worry about replacing it for a while.
As for a battery, a 7.4v lipo is PERFECT for any G&P. (or any airsoft AEG in general). They're compact, hold great charge, have awesome trigger response, and has the perfect amount of volts to rock the house at any game. And best of all, if used properly, they'll probably last you forever.
So lets recap.
G&P M4- new sector gear & MAYBE piston, and
7.4v lipo and charger (around $50). and you're good to go.
A couple good alternative companies are
Kwa, VFC, Matrix, G&G. These brands all have great full-metal external options, and great internals:)