$550 is a good starting point for a gun, especially if you're willing to buy used from a reputable seller.
Strelok recommended AK's.. I don't personally like them but they're solid guns. I would recommend other V3 gearbox AEGs, mainly the G36. It's easy to strip down and work on and the CA version with the nylon body is very durable (I bought mine years ago and it still works, body is still in great condition). 330 FPS stock and about 12.5 RPS fire rate on an 8.4v, higher on a 9.6v. Only think I hate about them are the magazines. KWA also makes a really nice G36c that will fit a large battery and has one of the best stock mechboxes in its price range, I just find the ABS body a little weak.
A G3 is another cool gun that isn't overly common. Uses a V2 mechbox but with a metal body they're tanks.
Another I might suggest is the P90. Lots of cool in a small package. It's a small gun though and doesn't work well for everyone and the mags are huge, but the V6 mechbox is shit simple to work on and get out of the gun. Generally P90s are very reliable.
You can always go with the classic M4/M16 metal body and a reinforced V2 mechbox. Not a huge fan personally because everyone and their mother, dog, girlfriend, and crazy roommate has one, but it's a versatile platform.
Basically find a type of gun you like then we can point you in the direction of an airsoft equivalent that suits your needs.
Haha, awesome. This thread should be stickied and used as an illustration to n00bs about the kind of answers they get when they actually take the time to ask a good question!
"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."