I just went through something similar and had some good advice on my thread (
http://www.planet-9.com/porsche-sal...vice-needed-cpo-service-history-question.html)
A few points from my experience:
- Carfax was useless and showed minimal info
- Dealer provided me with CPO checklist they performed
- I asked for the vehicle build spec sheet which they also provided
- I asked for a DME. They charged me extra for this (though strictly speaking it should probably be part of the CPO procedure). It was a PDK vehicle and most ppl say a DME report is useless as as a PDK vehicle can't do stage 3 and above overspeed, but I think it's useful as another point of due diligence to see how much of it's life it spent in stage 1 or 2 overspeed (e.g. at redline) and also total operating hrs to get a feel for average speed and type of driving, etc.
- Getting a PPI is a must regardless of CPO certification or DME report
- I would ask them when the original warranty ends and when the CPO warranty ends to get the exact dates
- The vehicle I went for already had the 2 year service done. Other CPO vehicles I looked at which were almost at the 2 yr service date didn't have the 2 year service done, so basically within 1-2 months I would've had needed to do the 2 year service. Keep that in mind, ask when the next service is due and which service it is. Then use that as a point of negotiation as others have stated.
Hope that helps!