bianchi san jose or pista, surly steamroller, khs flite, schwinn madison, just to name a few, should be near your $600 range at your local bike shop. the san jose comes non-fixed, but any of the others can be done up as a braked single speed. go talk to the people at you local shop and see if they have anything in stock that, even if it's not in your price range, should help you decide on sizing and features.