Never having owned a Porsche I know nothing first hand.
However, a friend took me to an independent who has a policy of his cars being serviced by an Approved Porche Dealer prior to sale.
Another independent with in-house service bay, sold him a 911 at £35K two years ago and he has had no problems with it.
Somone else I know paid £50K for an used 911 from an independent and the car failed one week later.
He then took it to an Approved Porshe Dealer who sorted a serious problem for £28K and would not release the car until the bill had been paid.
All a bit of a lottery it seems?
Cheers