[unimog]

PPI depends on you and the car. If it is a 'driver' perhaps not as much cosmetic stuff, but you want all the mechanical stuff covered.

Compression test, and if it passes that, then a leak-down check
Head studs (yup pull the covers, it is nice to look in there anyway)
Drive test of transmission, clutch, and starter ring gear
All lights and gauges (might be cheap bulb replacement, but you want to know wiring issues)
On that line, a GOOD look at all the wiring, for condition and if the PO has 'messed' with it (ie; radio, alarms, etc)
Wiper, sunroof, powerseats, window movement (power or crank) cruse control, AC (if equipped)
Glass (also rust around seal, or bad seals)
Body for signs of repairs, repaint, rust, and a detail look underneath for damage
Suspension; wear, all there, rubbing, weird changes
Interior, not only wear but also sighs of rust or water damage (pull the carpets)

I'd guess $200-$500 depending on shop and area.
Do not have the shop that maintains it do the PPI, a good change of bias (I'm a mechanic so no offence meant to mechs)

BTW get 'Porsche 911 Buyer's Guide' by Randy Leffingwell, it covers all the problem areas for every year.
Good Hunting,