We bought such a house (built in 1977). Always maintained by good craftsmen, but not modernized. Every one of our craftsmen said this was a good foundation. We have now invested about 18% of the purchase price and renovated everything so that it is visually up to date, but also energetically sound (but you can’t turn an old horse into a fresh racehorse!). Ultimately, the idea was that the renovation would last for the next 25-30 years (then we would be 75), and during that time we would live well. After that (or even before?), depending on health, I would probably be interested in senior-friendly accommodation and in selling. That’s how the previous owner did it. Until then, I am enthusiastic about a small garden where I can grow something, enough rooms, and a cellar bar in the style of the 70s. To get back to the costs… for windows (from D) we currently get within 4-5 weeks at an acceptable price (including removal, disposal, installation about €700 net per sqm, 4 units, triple glazing, moderate burglary protection).