Supposedly the open bolt version of the WE GBBR's are a lot better. So I'd get that if your going to go the GBBR route.
Still it's a good idea to pickup an AEG even if it's a cheap $120 Kraken or something just so if your GBBR dies during the day you have a backup to use.
As for number of mags. I've found that between 5 and 10 depending on playing style, game, and others are all you need.
ie. I'm used to going through maybe 1000 BB's over ~4 hours of gameplay but last weekend I went through a good 1500 BB's over 3 hours of gameplay just because everyone was playing differently than what I was used to and I ended up shooting a lot more for suppressive fire.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE