Smart 42 petrol or diesel?

Hello, my girlfriend needs a little town flicker. She totally lost herself in the smart, so I’ll inform myself here for her 🙂 My girlfriend is in training and has 2 weeks of work and 2 weeks of school. She drives every 2 weeks 2-4x daily 20km (1 times the distance). At the same time 70-100km/h around 95%. The other 2 weeks 2x daily about 10km at 100% city. Now I wonder to what extent I should grab a diesel here. In the gulf I know it, that a TDI is actually quite little designed for short distance (<20km) (so it will probably be with every diesel). Therefore the question to the speczzies, whether you should pay a diesel or a petroler. Furthermore, I ask you what is to keep up with the offers for the 2,500 EUR - 3,000 EUR. Much more he should not cost. And still the question, what we should pay attention to when buying in general. I thank you for it! Greetings And what for God's sake with the engines? Look at the offers only "motor damage", etc.