You're doing well. At age 50, most people are slowing down, and their bodies are deteriorating, having long since abandoned strenuous physical exercise due to the various forces in their lives pulling their time and focus away (kids and career, typically).
The thing to realize is that your body adapts to whatever conditions are placed upon it. Regular martial arts practice will keep you in shape for the rest of your life. And there are many examples of people who kept on practicing martial arts well into their 80's and 90's.
If you're doing martial arts, your body will adapt to that and will become flexible, fit, and strong. If you go from sitting at a desk to sitting on a couch every day, your body will adapt that and become fat, rigid, easily fatigued, weak, and sickly. It's just that simple.
So you have the right idea.
Next, you mentioned having arthritic fingers. Sorry to hear about that. That might make it more painful for you to do anything grip related. I can't be certain of that, but that's what I'm guessing you would feel, especially decades from now.
So that means you should be more concerned about grappling arts, because the grip is highly stressed in those arts. It's not uncommon to hear this as a complaint of people training gi (uniform) based martial arts like Brazilian Jiujitsu or Judo. They get their fingers firmly wrapped up in their opponent's gi, and then as they thrash around, their fingers undergo a lot of torque which can cause sprains. They tape their fingers to help with this.
Read this article to get some background about why some BJJ people tape their fingers: http://jiujitsumag.com/finger-tape/
Again, I'm not sure that in your case this would be a big problem for you. You might never have an issue with it just because you're going to be more careful and aren't going to approach grappling arts at full force like a lot of young competitive grapplers do. And your partners aren't going to go overly hard on you if you're not going overly hard yourself. It's something you would have to experiment with by trying a BJJ class for a little while. You won't know until you try. Just don't sign any long term contracts.
Your arthritis might also exclude weapons based arts like Filipino Martial Arts (escrima / kali). Their stick fighting practice involves holding onto sticks very firmly while whacking them against opponents' sticks. That puts a lot of force on your hands, forearm, and fingers. Your fingers often get hit by accident, also, and it's not uncommon to have a finger break on occasion.
As for percussive martial arts like Muay Thai, MMA, Taekwondo, Karate, kickboxing, and boxing, they practice punching on heavy bags. This will put forces on your fingers and forearm. Again, depending on how your body reacts to it, it might be either bad or okay for you. You're going to need to experiment by signing up for a short period of time and trying it out.
Wing Chun practices with a wooden man dummy that involves hitting your hand against firm wood. Depending on the school, they might want you to really hit it hard, or they might be really soft. This is highly variable, so you have to look at what's going on in each school and talk with the instructor beforehand.
Aikido uses the grip, but the torque tends to be on the wrist instead of the fingers. In general, I think Aikido is a lot softer than most grappling based arts. And Aikido has a large percentage of older adults in its ranks. This might be your best bet for your fingers.
However, I have also observed a relatively larger percentage of out of shape practitioners in Aikido. So you're going to have to gauge that by looking at what kind of shape the black belts in each school are in. If you see a lot of overweight, out of shape black belts in a particular school, that's your answer. And that goes for any martial art school you come across, not just Aikido.
All of the martial arts you listed could present issues for your fingers in various ways. All of them generally improve physical fitness. So my recommendation for you is to approach it by first figuring out which martial art appeals to you the most. Don't worry about your fingers too much. Try it out for a few months, and then decide if it's something that you want to continue. If you're finding that your fingers are having problems, then you'll maybe want to look elsewhere instead.
Hope that helps.