You're not spending a lot of money, as mtbs go. Think about single speed. You shouldn't have any trouble riding one gear in a flat place and you may discover it's a lot more fun when you don't have to think about what gear you're in. You can put the money you would be spending for gears, shifters, etc. into a better quality frame and wheel set. In that price range I would look on Craig's List, Ebay.
Good luck