Smart CDI the right car?

Hello, I would like to buy a used car soon and have put the smart cdi into the narrow selection. Now I want to know if the smart is the right car for me. I am often at risk of the smart CDI and am satisfied with the driving behaviour. As a matter of fact: I have about 7000 euros available which I would like to invest in a used car. I will drive about 300 km of city traffic every month, 1000 km of motorway (with approx. 120 km/h) and 1400 km of highway (I think a diesel makes sense when it comes to mileage). I don’t have a lot of money, therefore the car should be as imperturbable as possible, or not often what expensive things are going wrong. I have never been allowed to drive a car on me, and I am only 23, which is why the insurance is a big cost factor. Is the smart CDI a good choice, if it is important to me above all to keep the operating and undercrow costs low. The consumption values of the smart speak for myself, however, I have already read here that the fact that the actual consumption can differ greatly on the highway. Which vehicle’s a good choice?