Do you have the resources to retire him if he goes lame? Are you okay with a horse that only can do light work?
A friend of mine takes in free horses all the time. They all have physical issues and are all over the age of 20. She just does light rides and brushes them daily. She says she doesn't want to have a horse that will outlive her. I don't think any of them are sound, but it is good for their minds to do at least some light riding.
Both my horses have lameness issues and I have health issues so I rarely ride. I miss having a horse I can canter and do faster stuff with, but with my health issues, I don't need to be pushing myself. One of my horses can only handle a 20 minute walking ride (just to help her arthritis as she gets stiff otherwise). My other horse can handle slow trail rides, but can't canter at all due to lameness issues. Both are super safe though, and to me that is the most important thing.
You could try a course of Pentosan injections which runs about $200 for a years worth for one horse. I can't afford joint injections, but do use the Pentosan and I think it helps for the arthritis. I keep bute on hand as well and use that before farrier visits or if the old mare seems more sore than usual.
I would suggest visiting the horse, see if he clicks with you, ride him, and see how he is. Plan to visit the day after riding, and see if he gets worse. Get a vet's opinion if you are really concerned.
You don't need to pump a ton of money into him- just try to keep him comfortable or pasture sound. Just be prepared to retire him at some point in the future.
Do you have pasture for him? Arthritic horses do very poorly when stalled. The other issue is can you ride him consistently?
My old mare does best if ridden at a walk for 20 minutes 3x a week. The vet thinks she has arthritis in every joint by now. Such is life when you are old, but she is happy.