I don’t have a Friesian, but I did buy my warmblood gelding at 15. Knowing that he was in his mid-teens and stepping down from a demanding career in the Big Eq/ High Performance hunters, I went in with my eyes wide open.
Maintenance was going to be required to some extent. He was going to need more careful management with feed/farrier/dental care as he got older. I was prepared to deal with those things, because a horse with his level of training and experience could teach me SO much. My riding has improved tremendously in the last three years that I’ve had him. He truly is my once-in-a-lifetime horse, and I’ve often said there isn’t much I wouldn’t do for him.
My best advice would be to do a thorough pre-purchase with a reputable vet. X-rays, bloodwork, flexions, teeth, etc. When I did my PPE for my horse, my vet gave me a good piece of advice: every horse is going to have something show up on a PPE that could be a potential red flag. But the red flags for a Grand Prix horse are going to be very different from the red flags for a horse doing the kind of things you describe. So make sure your vet is considering the PPE results through that kind of lens too, if the exam comes back without major issues.
You say that he seems as though he hasn’t had consistent work in some time. Getting a 17 year old horse fit for regular riding will also be a considerable challenge that you’d want to discuss with your vet.