Buy on the car's maintenance history. These cars are at least 10 years old and it is nice to have someone else have paid for some of the things that just age in any car.
Buy on the seller's motivation to sell. Why are they selling? If you can verify the answer to that question, you can eliminate scam artists.
Buy with the advice of an experienced Porsche mechanic. If the seller won't facilitate a PPI, then walk away. There are hundreds of these cars for sale today.
There is a list of changes year to year
posted here.