Some terminology issues here…vets do not proclaim sound or unsound. They deem them “suitable for intended purpose at this time” or “ not suitable for intended purpose at this time”.
Sounds like OPs intended purpose is a couple of 3’ shows a year, assume some lessons and prep for this, and cannot afford more. Least that is what I glean from what OP has specifically said.
Vet has communicated this horse is suitable for this purpose at this time. That does not mean “sound”, none of them are “sound” at 12 with a performance career. But they can be suitable for a lighter job which I think is what they mean about this horse.
If OP can afford the purchase price, has a guaranteed place to park him for the next 15 years should he become unable to continue even the light work? And afford another horse to ride while he heals or retires? No reason not to proceed with the purchase.
For me, this was never a question as I could only afford one good horse at a time and saw the last one through retirement without replacing it.
Might suggest a second opinion from a neutral vet who has no relationship with seller or trainer. Images can be easily transmitted to specialists these days, no need to haul to a clinic or pay a farm call.
Me? I would want some current, as in now, imaging on that suspensory injury, those are notorious for incomplete healing or reinjury, especially if they were a repeat of even earlier issue. Age related arthritic changes and job related tread wear are unavoidable but can be managed…but there is a cost and that has to figure in to any decision to purchase.