Ok, then vet sounds good.
Wait until he has seen the horse and be prepared to walk away on his recommendation. Also talk to him about the condition etc.
would you be happy if the horse got livelier and tested some boundaries?
On the other hand, if the vet is happy with the horse for what you want to do (this doesn't necessarily mean a full pass as he is quite old), it should ease the questions in the back of your mind.
Also, being brutal (sorry, I know I keep on being miserable), depending on his underlying fitness/soundness and conformation, you have about 4 years of the horse at his current level, approx 5 at slightly gentler level and then from 25ish in a gentle retirement level of work e.g. Hacking and low level dressage. He also won't be able to be sold easily, which may not be an issue if he is your forever horse. Plus insurance can be a bit awkward for veterans.
Having said that, I know some Pretty serious dressage horses still throwing shapes at 25yrs, so it does depend on fitness, sounded, how they've been kept, etc (and sometimes dumb luck avoiding injury!). And better to get an old horse that does everything you want than a young one who doesn't know what you want it to do.
Anyway, the vetting should reveal all and help you decide what to do.