Getting the precise correct size and fit is critical for safety and enjoyment. For around $200, you can find used bikes that sold for $1,000 ten years ago. But, unless you buy that used bike from a good bike shop, you might also be buying concealed problems with the frame, fork, and components.
Right now, most bikes shops are doing "clearance" sales on their 2006 model mountain bikes. Trek, Specialized, and Giant have some very nice mountain bikes with a list price between $250 and $399 that are currently selling for between $200 and $300. With these bikes, you can get a precise fit, a life-time warranty on the frame, a tune-up before delivery, and another tune-up free in thirty days.
Buy from a shop that is within a mile or so from home. That makes it easy to get the regular small adjustments to the shifting and braking that are the difference between an "okay" riding experience, and a great riding experience.
Buy an OnGuard Pitbull to lock around the rear wheel, and a $20 cable lock for the front wheel. And, don't leave the bike out of sight, even in "safe" areas for more than an hour or so.