The B50/51 would be a good choice, you might also be able to find an older AA S400E or an Air Force Talon in that price range. If you can wait a few months, the new Benjamin Marauder will be out, I believe it's listed at $500. Shrouded, 10 shot repeater, 2K psi fill, if it ends up only half as good as it sounds it's going to give some of the "Big Boys" a run for their money. Best bet would be to have cash in hand and keep a frequent eye on the various Classifieds, here, YF, and Brad's are the ones I use. Come to think of it, all 4 of my current shooters came off ad's on Brads's. Let me know if you need links.
Dave