With 70000 km, I wouldn't buy a Cayenne Turbo privately from an owner, only from an official Porsche dealer.
Why don't you talk to him to offer his car through his Porsche dealership? This might cost 2000-3000 Euro more but you get a Porsche used car warranty which is worth gold if something happens. 70000 km are a lot, especially on a car which is almost new on the market. I wouldn't risk it without a one year warranty.