Questions about PPI

[Surfncpa]

I know now that not having a PPI done before making a used car purchase is really dumb. Having one done would have saved me a lot of grief and $. I have learned my lesson.

Having never purchased a used car before this Boxster, I have no clue how a PPI even works. I assume the mechanic you hire charges a flat fee? What is the normal price range for that service? Does the mechanic go to wherever the car is or do you need to arrange to get the car to his shop? I wouldn't think a dealer would be very accommodating either way. How much time does a mechanic need to do an inspection? What kind of problems could there be that the mechanic wouldn't catch? What else does a clueless person like myself need to know about the service?