Anything from the AR15 or AK families (as well as the G36 and MP5 to lesser extents) will be perfect as those are the guns with most aftermarket support and upgrades/accessories. They all run on a V2 or V3 mechbox.
If you want a GBBR you should probably plan on spending at least $1500 minimum to get it to "gameable" state. That is unless you decide to get something like the KJW/TK M4 in which case they're pretty good out of the box.
You should really just spend your money on one good primary for now then later down the road next year or maybe next paycheque but a pistol. ie. don't buy quantity, instead buy one really good gun instead of 2 not so good ones.
As for retailers I can't tell you where to go since you're not verified but it's not like it's hard to find that info if you've already seen the FAQ's. Work on getting verified it's definitely worth it, I've bought like 99% of my airsofting stuff from the classifieds.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE