Ditto to the "get an exam". I have a friend (very experienced horse people in her mother and her instructor) who bought a mare for eventing. Lovely horse, no sign of unsoundness at all... they put her into eventing work and she broke down within six weeks, and now sits in the paddock doing nothing because they don't think she'd carry a foal, and have recently gotten out of breeding due to a series of disasters. They had a PPE but it was without x-rays, which would have been needed to pick up their mare's unsoundness. This was an expensive mare so if it was me I'd have had x-rays done, but that of course is only after having heard what happened to my friend. Before that, I didn't think it was really worth it.
Re the "which vet to get", probably one in the horse's area, but not the one the owner uses as they might be biased. My vet would NEVER travel more than an hour in any direction, and if he did it would cost a fortune just in callout fee! If there is only one horse vet (SPECIALIST vets - large animal vets often don't know horses beyond the medical problems and how to treat them) in the seller's area (or none at all) then just make sure you take someone experienced with you, preferably someone who knows how to do flexion tests and ALWAYS someone who knows about conformation. For an eventing horse, where I am, the PPE is a flexion test on each leg and a general health and soundness exam. The vet will also look at conformation if they are a horse vet as it is a good guide for long-term soundness.
I have never had a PPE done on a horse I have bought so this is kind of a case of do as I say, not as I do - BUT, I always have experienced help with me, and that experienced help is an ex-vet-nurse with 27+ years of experience with horses. If I was selling, I would encourage PPE's, regardless of the value of the horse.