You can actually try look around for a Giant/Scott, or some other lower end bikes to your budget. Getting a branded bike is definitely better than a Kmart bike. Besides, if u like biking after some rides, you wouldn't want to spend a lot on upgrading since a typical Kmart bike may use very obsolete parts.
If money is really a problem, you can stick to a hardtail mtb with rigid fork, with at least a Shimano Alivio/Acera groupset. My 2 cents worth, but hope it helps.