I couldn't make up my mind either which is why I now have an M4, SCAR-L, M249, AKS-74U, SPAS 12 and have started looking at M14's. There's always something else I want....
and need to get some trousers + carrier so i'm trying to stretch it out as far as i can go (saying that i really want the interceptor vest/carrier i've seen on taiwangun)
NERF or nothing just ask JBBG - fantastic !!!! Seriously won't go far wrong with a G&G M4 perhaps - the model - hmmmm, but at least it should be good start with easy upgrades, maybe a side arm or grenade and a basic loadout
Good solid choice. Certainly plenty of aftermarket goodies for the M4. Don't go mad with a sidearm until you know whether or not you actually need one. Grenades - start off with paper pyro and see how much you use. Spend your money on spare mags and decent basic, functional kit and you won't go far wrong.
For a primary I'd go new, you never know how worn out second hand stuff is and you don't want one of your first skirmishes ruined by a stripped piston or such. Then if you want a cheaper second gun, try preowned!