I think you are worrying too soon - If he has all these admirable traits, and you haven't put money down, and he is at their house, and you have a vet coming for the PPE, you are fine.
And as was suggested earlier, if his PPE does reveal a "catch" that does not affect his riding ability, consider asking for a price drop to reflect his unsoundness and take him anyway - if he can do everything YOU need him to do, who cares if he won't stand up to high impact events? Every horse has a glitch, whether mental or physical - it is up to you to decide if this particular horse's glitch is one you want to deal with.
Don't worry - just talk to your vet about your concerns and then go from there.
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela