Try to find one thats never been down. And doesnt have that many miles. Not that easy to find for your price range, but theyre out there. Cycletrader.com has a bunch of good deals.

Also take a look at the teeth on the sprocket to make sure the teeth are not sharp/pointy. I believe that if they are, you would have to change the sprockets soon. They should be slightly rounded if in good condition.

Also, you might not think about it, but a good set of tires are $250+, so try to find one with decent tires.