I do not know enough to comment on your joint health... but when you are riding make sure your seat height is properly adjusted (too high or too low is bad for your knees) and take it pretty easy to start with - easy gears are your friend.

As for bike choice, the only things you need to be concerned about are bike fit and wheel quality. Any standard good quality used bike will likely do fine, but yor wheels might not last long. But if you spend ~$100 on a 10 year old mountain bike, $50 on a tune up, then have the rear wheel professionally relaced after spokes start breaking, you should have a reliable bike.

Also - if you happen to find a great deal on a super light bike, don't get it... super-light bikes designed for racing are the only type with frames generally not suitable for reallly big riders.