If you go back and read the responses by the many people who have written in answers to questions just like the one you pose one thing that often comes up is avoid entry level bikes with suspension forks. Having a lockout helps a bit but cheap suspensions don't work very well, add weight to the bike, and takes away from spending that money on better components elsewhere on the bike. It is nice to have a reasonable number of speeds on a bike so that you can find a gear that is comfortable (proper cadence) for the terrain. If you live in flatland, fewer gears are needed than if you have hills to climb.

If there is a bike club in your area you might contact them and ask if there is a knowledgeable recreation rider (non-racer) who would mentor you in finding a decent use mountain bike. That person would know the gems from the garbage and if the price is reasonable.