A well-maintained used Porsche is one of the great used car purchases — the 911 depreciates slowly, the Cayenne is outstanding at high mileage when serviced, and the Macan is a brilliant compact SUV. But Porsche ownership demands either deep pockets or discipline: a pre-purchase inspection by a Porsche specialist is non-negotiable, IMS bearing on older 911s is a known risk, and PDK gearbox fluid is the one service item that owners skip at their peril.