Purchase advice

Situation: Drive more than 30,000 km a year and currently have a 5.5 year old Opel Corsa D 1.4 eco petrol. I am looking for a diesel to not always refuel the more expensive petrol at the kilometers … Unfortunately I am a complete layman in the car area – with me it only has to drive without problems. Do not put a value on “shrinks”. Of the 30,000 km a year are well half on the highway. Driving behaviour are there 120 km/h and then turn on cruise control. Do not put a value on a lot of horsepower – a never It is definitely more important to me that I always have a look at it and sometimes also annoy my girlfriend with the invitation not to heat like this or to drive foresight at times^^. I have now looked around at mobile and in the sewing focus are the Opel Astra, Renault Megane, Hyundai i30. Requirement: max. 3.5 years old, max. 60,000 km run, diesel, air conditioning, like a combi, but definitely a 5-door. Budget ceiling is 11,000-12,000 €. My Pro In particular, it is important to assess which car fits my requirement of low consumption. Especially in the case of engine capacity etc., all the different versions confuse me. Which engine/car fits my driving behaviour (fuel-saving and yet enough hp to be able to overtake on the highway at 100 km/h times n truck) or my route 45 % highway – 25 % highway – 30 % city center. In addition, where the quality differences of the individual brands lies. I hope you can give me here a I would also like to accept alternative proposals for the three cars mentioned above.