Smart 451 Cabrio petrol or diesel? Private or dealer?

Hello friends For the city for short distance I would like to buy a smart of the second series (type 451). So cabrio. Budget 4,000 euros, as much as possible worth-stable car. I want to keep the only 1 year and have no problems during this time. After that, a company car can. Now there are online interesting offers of 3,000 – 4,500 euros Me is very important as far as possible to get no problems with the car. I want to avoid workshop costs. Therefore I only look for cars, the checkbook are maintained. Should I only buy from the dealer because of the warranty? Which engine is reliable to get no problems? I have read in the car picture that the petrol engines often only produce 100,000 – 120,000 km of running performance. The diesel should be very reliable and otherwise unproblematic. In forum contributions, however, there are contradictory statements. Which engine is the right one? Should I buy from the dealer or suffice from private with the checkbook? And why are there so few in the combination Diesel and Cabrios? In Hamburg and the surrounding area a handful.