Purchase advice First car, approx. 20,000 km/year, lots of highway

Hello, since I will start my training in autumn and want to commute on the weekend, I am looking for a suitable car. Every week I will drive about 450 km, later more, so that I will drive 20,000 km a year, probably highway. Since I will only become 18 shortly before the start of my training, the insurance is expensive accordingly. However, with the insurance as a second car over my parents SF 1/2 is possible. I have a budget of up to 8 000 Euro available, nice would be e, if the running costs (insurance, fuelling) are also relatively cheap. I am looking for a not too small car (Polo is too small – rather golf size), which is suitable for long regular motorway rides and is also overall very reliable (and best still economical ). It should definitely be a diesel. After extensive research I already had the eye of the Audi A2, built in 2004/05, since it is especially fuel saving and insurance favorable. However, sin d the most recent models now also at least 11 years old and have mostly run around the 170 thousand – is it recommended to buy an old car, since the reliability is so important to me? Besides, I like the Ford Fiesta from 2009. Does someone know about the brand and can tell me if this car is suitable for my needs? Or an Audi A3 , here speaks to me especially the design and the size. However, here the insurance is not exactly cheap and I know I’m not at all interested in whether this car is reliable and what kind of shit it has… I’m grateful for every advice, I’m also happy to accept other suggestions! Lg bille08