For the 2 cars I've bought from private sellers (986 Boxster and 987.2 Cayman) the seller took it to a shop of my choice (but that the seller agreed on as well, no sketchy places), the shop did the PPI, and in the case of the 986 I flew in and drove the car home directly from that shop (who I had then hired to do the IMS bearing).
If either seller had balked at taking it to a shop for a PPI I would have walked.
Do you have a trusted Porsche shop in your area? If I sold a car I would be happy to take it to a shop, but I would be the one driving it there and back and not the potential buyer (who would be welcome to take test drives, but with me in the car
)