I am always also eyeing the saved 200k, which could be well invested in travel.
For your retirement provision, it is definitely three times better to acquire property now. I would never even come up with such an idea in my dreams. Rents will rise and your installment remains constant. I would definitely tend towards the existing house. With a new build, you are easily 200k over the price of the existing property with renovation costs. Because with a new build, there is always something additional (upgrading, house connections, soil survey, etc.). With the existing house, you have now taken action and I find 1995 still completely okay in terms of age. You have good building substance. And with the cellar, there is enough space. And if a room is missing: there is always a compromise in house building. Everyone who has ever found a room, the plot in the new build, the price, or the location not optimal. There is always something. Otherwise, you never find anything and always stay in the rental apartment. And the price also seems really fair to me, as long as it is not the flattest land (30 km radius just nowhere). And you don't want to be landlords in your own house. Then you also have to argue with these people. This living situation would already be unbearable for me. Therefore: buy the existing house and be happy. That is fate.