I have owned a few over the years, and still ride regularly. The above advice is good; lots of great used bikes around for much less money than new. One thing I would add; if you have a choice between disc brakes and the old style pad brakes, go with disc. And, cheap components will be a constant pain in the neck, either breaking or needing constant adjustment.
Don't bother with the Wally world bikes; they will not hold up to any sort of serious riding.
Having said all that, riding a mountain bike off road is one of the best ways to stay in shape; training on hilly, mountainous country will get you ready to chase elk if done often and intensely.