Buying advice Mini Cabrio

Hello, my wife wishes for a Mini Cabrio and after I have absolutely no experience / idea of the Mini ́s I wanted to ask once in the round what you should pay attention to when buying. The Mini should be up from year 2015 up to approx. 40-50tsd. Kilometer. With the engine we would be free in the decision, can also be a diesel. With the engine would be only important to me that he has little problems and of the performance is sufficient to have some driving fun. Thanks already for your Experiences. Greetings Stefan