Look for something that has been serviced to at least some extent and, in your price range, I suggest focusing in on Denon, Sony, Technics and Yamaha, all of whom made very good affordable decks. You should, for example, be able to pick up a nice example of a Sony TCK-611S, which would be a nice option.
Belts are the main problem with older cassette decks, but also heads usually require at least a clean using isopropyl alcohol, though this obviously won't take away the effects of wear.
If you want something really special you would need to save quite a bit more up and look for a good Nakamichi, but I think that's more of a next step should you really get back into the format in a big way.