If you've got $600 to spend, the first thing I'd do is cruise the crowd for at least a couple of hours. Who knows, you might find a good deal on a preban, or for that matter someone with a postban and save yourself some money either way. But make sure to go with cash in hand.
Other than that, unless you like those thumbhole stock Norinco's you'll probably find mostly the Romanian or Egyptian Maadi's. Both are generaly good rifles for the money, but make sure to look them over for crooked front site posts and such. If it looks good, then I'd recommend buying one. For the money they're great. Though I wouldn't pay more than $375-$400 for one.
I mean, sure you can pay more for an AK, but why?
Though I have to add, if you come across some of those Arsenal AK's (made in TX) I hear they're pretty nice. Just haven't heard how much they're going for.