Whenever possible I would use another vet. I have only dealt with this a couple of times, but I was fortunate enough to have a college with a great equine program closeby-I called them to see who they used. I have had friends go through this too, and they found a few farms nearby with great reputations, so they called and asked them who they used. Just explain the situation, but not the specifics of who the seller is, etc....and usually they will tell you. We have had it happen to be the same as the sellers vet...but only used them when others say they can be totally unbiased, etc.
We get a standard vet check and we get the horse drug tested, regardless of how much we like or trust the seller. Although it is hours away, I would try your hardest to be there for it and absolutely don't use their vet. Ask your current vet if they know another in the area or find one yourself. We also ask for old medical record copies or for them to be released to us, most sellers will happily hand them over.
I would not use the seller's vet if you can avoid it. Too much chance for bias. Maybe you can ask the seller to record the exam on video if you can't be there. It might be a little more telling than just a written report from the vet.