I recently had a PPI done by a dealer on a Porsche 993. They did a fine job, but it wasn't quite as in depth as I'd have liked. I didn't have much say in the matter as to whether the PPI was done by a dealer or an indy shop due to logistical issues with the seller.
That said, indy shops will do a great PPI to provided they know these cars well and are a reputable shop. Their PPIs might be a bit cheaper or more thorough than a dealer, but it is hard to say in generalities.
I had a lot of variation in PPI quotes amongst dealers when I was looking too.
Lastly, I've got a well maintained '99 with 82k miles on it for sale.
Check the for sale section of these forums.