I still don't understand
 

How can a PPI serve any IMS cause at all unless the car has documentation from a shop that replaced it and you can call and verify from a known good wrench ?

The rest I understand, belts, waterpump etc....but IMS how can anyone look through the engine and tell you anything really ??

I'd put the cutoff at $3K.

If you're buying a Boxster, any Boxster, for $3K or less, the price alone tells you that it needs $5K of work. At least. You don't need an inspection to tell you that.

A PPI should include a removal of the oil filter and an examination. Not a sure bet as the oil could have just been changed but better than nothing and you presume some driving to get to the PPI location that might spread debris to the filter.

A PPI doesn't just focus on the IMS, mine included two pages of dense facts, measurements, opinions, etc. And it was done before the IMS was a known "look at".