I would recommend going to your local bike shops and doing some test riding of bikes that fall in your price range, unless of course you are willing to not only purchase the second hand bike but also take it in to a shop to get it taken care of (tune up and what not). of course this is assuming you're not comfortable doing the wrenching your self.

At a bike shop they'll be able to fit you to the right size bike... sure you can ride one that's the wrong size and/or geometry but it won't be as comfortable for ya.