If you actually plan on using a pit bike for the pits on race day, or to cruise around the local swap meets/flea markets, Id get an old Honda Trail 50 or Trail 70. I used to have a Mini-Trail 70. They are very cool little bikes that can be made street-legal in some states and are still fun in the woods.
That said, if you're gonna be using the bike almost exclusively offroad, I dont have any experience with the newer little pit bikes, which are probably what you would want. Those old Honda's don't like very rough terrain or being jumped. Nowhere near enough suspension.