It's summer, check the garage sales. Keep an eye out for bike brands you see in bike stores, not in CT or Walmart. As for "casual" mtbing, I'd say cyclocross at a minimum, but if you want to jump logs and do drops, you might want a mtb instead and just swap slicks for knobbies as needed. If you look at the CT mtb's, there's a sticker on the top tube that says, "Not for offroad use" or something to that effect.