If it is a reputable dealer, then yes, I would trust what their mechanic says is the current condition of the car. A reputable dealer should also back the car up with a warranty.
A PPI is usually performed by a 3rd party with no interest in the vehicle. It is something you pay for. Usually when you are buying a car from a private inividual. If you don't trust the dealer than you should seek this avenue. Having said that, if you don't trust the dealer than why would you buy a car from him in the first place? An individual who is selling his Ferrari has something to gain by hiding imperfections, a reputable dealer who makes his living selling these cars has nothing to gain by hiding imperfections. An individual doesn't care if you tell your friends you were happy with your purchase. A dealer does.