Even with prices dropping, $8,500 sounds very low. Definately get a PPI to make sure they are not trying to hide something. If everything checks out, you got yourself a great deal.
The engine was replaced by a Porsche dealer and the car has been maintained by Porsche with all records? If the car drives good and feels good then i don't think you have anything to worry about. I've logged just over 100k on mine with very few problems. So, if it feels right, go for it.