Before you go too far down this path, I would have a candid conversation with Jonathan--who is an expert MTBer--about whether any of your physical issues, namely your arthritic hands, will be a significant issue for mountain biking. I'm somewhat concerned that they'll be a very limiting factor to your ability to get out of the saddle, jump, hop, etc. My somewaht limited understanding is that it's just.a much more physical discipline.
Now I say that as a non MTBer, so take it for what it's worth. In the very least, I'd consider renting a MTB before you put moeny on the table for one to make sure it's for you.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher