I understand how you feel but like you said you don't have a choice.
When I was 20 I took my car for a service. It was less than a year old and the mechanic charged me well over $400! (change plug leads, filters, oil change) This was in 1986.
I vowed and swore that would never happen again and so far it hasn't but only because I do all the work on my own cars.
The only thing I go to a pro for is new tires.
Back to you... I guess you have no choice but to trust someone.
As a suggestion ask at your local Porsche club.. See if someone can recommend a good mechanic.