Classic Army M4s, IMO, are some of the best out there. I would take one over G&G any day.
However, remember everything you're going to need:
-Additional batteries ($50)
-A good battery charger ($70-$100)
-Eye Protection ($70-$100 do not in any way cheap out here)
As for a GBBR, expect a setup to cost you a good $500. Lets say you go with a basic M4, you're looking at about $350, plus at least 5 magazines at $45 each ($225) -GBBR mags hold 30 rounds, one won't get you very far. With 5 heavy mags you'll need something to carry them, so a good LBV w/pouches is probably going to run you in the $100 - $150 range minimum.
$350 doesn't go very far.
"Someone in a Prius tried to race me at a stop sign the other day. I couldn't believe it. I had him for the first 100 feet or so but I can only walk so fast."