My trainer always asks the seller who their Vet is, just so she can use a different one. If for no other reason than a Vet who is familiar with a horse/seller could potentially miss something because they are going into the situation with an "oh yeah, that horse, she's perfectly fine, I'll just go show the buyer that" where a Vet whose never seen the horse before is going over it with a a fine toothed comb to ensure the seller isn't busy trying to hide anything since they don't know the seller either.
At my trainer's suggestion I went with an extremely large Vet practice that has a teaching hospital for my Vet check last year since we lived hours away from the seller and had no clue on Vets there. We were beyond thrilled, we had an older Vet come out who'd been around the block more than a few times but was also extremely up to date on new treatments and findings as their hospital gets the "my Vet is stumped, this is our last hope" cases. She brought a couple students along with her and we had one very thorough exam.
I paid $800 but had a series of x-rays done on one leg that failed the flexion test and that nearly doubled the original cost. As far as what it covered, pretty much everything. I don't think there was a square inch of my horse that the Vet didn't poke, prod or violate. She had the horse walk, trot, canter on a variety of surfaces, indoor and outdoor, flexion tests, heart rate while resting, working, took his temperature a few times, checked his vision, his teeth, you name it, she did it. She spent about 3 hours total with him. Oh and we did do some bloodwork as one heck of an unspooky under saddle 4yr old, we wanted to ensure that was his temperament not a chemically induced one and we needed blood to obtain a current Coggins any ways.