This is the draft cross, right? The cresty one?
I would expect to pay the entire cost as the buyer of the horse, and I'd also expect to dictate exactly what gets covered by the PPE. It would be one thing if the seller was promising you that the horse does *not* have a metabolic disorder...but it doesn't sound to me like the seller has actually explored this route - sounds like he just assumed that the horse pudgied-up because of inactivity and excess feeding. If he was promising you this, it might be reasonable to ask him to foot the bill for a test (or to give you the results of the test that *he* had performed, as a part of the horse's medical records).
In this case, it sounds like the questions about a possible metabolic disorder come from you, so probably the test will also be out of your pocket.
Everything is negotiable, though - if the owner is conscientious he might be willing to split those charges...that way if the sale falls through due to a metabolic problem, at least he'd know about that and be able to manage it better than he has been.