It's a risky choice. You don't mention which engine you're considering, but you could face anything from electrical gremlins to catastrophic engine failure (particularly on V8 models). They're quite hard on consumable items like the tyres and brakes, too. Basically, a 15-year-old Cayenne is a complex and expensive SUV that's reaching the end of its serviceable life. Even covering only 1500 miles a year, you'll have to fork out for some hefty maintenance bills - or be prepared to send it to the scrapyard if it goes expensively wrong. This is one of the few times we'd recommend an old Range Rover as a more sensible option...