Getting another opinion is a good idea. When I bought mine, the dealer had "their" PPI but I got another one from "my" dealer. I viewed this as cheap third party verifications. I've noted it before that I also had a comp test and a DME download. The comp will tell you an up or down cylinder and overall health of the chambers and components related to the chambers, and a DME will tell you how many hits that unit has had in its lifetime. Especially valuable for 987 model units as it shows ignitions from 7300 to +9500. For 986 it shows hits from 7200 to +7800.
Even with all this information, there is definitely no guarantee that everything is ok. I brought my car in for some little issues recently and warranty confirmed 3 other components that required immediate replacement. The roadtests, visuals, magnets, etc. could not uncover these problems, only driving the car for a few k miles could.
A used car is a used car, caveat emptor!
I've also noted that 2 other dealers tried to sell me lemons that I was lucky enough to determine through roadtests, visuals, and questions. Oftentimes, salespeople will not be informed of specifics to prevent them from being in a position to misinform you as the buyer. The sales manager may know though, but they normally don't want to talk to you, possibly because of this reason, and the normal sales related reasons.
PPIs are useful, especially to the "non car guy", but even the car guy should consider this. But as I noted, its use is somewhat limited. Be careful, only you can look out for #1. Yes other gov't test are useful info also. The more independent sources you have , the better - for sure. CarFax useful too, and maybe one of your bodyman friends too.
Hope this helps you and others.
Cheers and good luck
