Vet checks are fairly reasonable - especially when you consider the cost of potential issues. You're not paying for tests, just a soundness and overall health check. You could even ask for a student or assistant to come out. It would be cheaper and the intern would likely be glad for the experience.
I'd ask for a vet and farrier reference, as well. If they're responsible horse owners and aren't deliberately hiding anything, they'll provide both. Look at it like hiring a new employee. You wouldn't hire someone on without checking their references!