Our first house - your advice is needed!

By "quantity counter" you mean those small great things hanging on the radiators, right? Quote from father "you don't want to have that thing hanging on the radiator, do you?", how to interpret that statement, I'll leave up to you.

Only your rating is relevant to you.

Regarding the underfloor heating. Would one be recommended in the basement? My parents prefer a warm floor and like to have warm feet.

A modern underfloor heating is at most less cold, but not warm. Warm floors would be unhealthy. Put down a carpet.

You might be overextending yourselves with the project, regardless of finances. You already have individual conflicting interests over trivialities like the old heating. How this will work out for you long-term and what might still come, only you can judge.

Of course, my post contains errors and is not technically perfect. I am neither an architect nor an expert assessor in this field. I have little to no knowledge of the subject, and I admit that openly. That's why I am turning to this forum and have asked you for ideas/approaches/suggestions!

Some opinions have been expressed that will make me reconsider. Wonderful approaches have also been provided that I will definitely take to heart and will engage with accordingly!

I think I went a bit too far with the nonsense, that was not correct! I apologize for that.

That clues, in a certain density, are almost evidence.

I am sorry. Please spit in my face, I do not understand what you mean by that.

Neither technically nor contractually do I see any reason to split the total heat generation into several small systems. In a multi-family house (not just a full apartment plus a granny flat like here), this is done because the neighbors are usually not related. Even social welfare offices have no problem with distributed costs. If the father already announces that without separate meters there will be quarrels, I would take that as a warning sign.

I'm sorry. Please spit it in my face, I don't understand what you want to express with that

In this year of construction, it would take the devil if room dimensions in BRM (or quarter decimeters) instead of BRM +1 and wall thicknesses in BRM instead of BRM -1 (or internally non-load-bearing 10 instead of 11.5) were not due to aerated concrete plan blocks. If that is not aerated concrete, "I'll eat a broom."

In this production year, it would take the devil himself if room dimensions in BRM (or quarter decimeters) instead of BRM +1 and wall thicknesses in BRM instead of BRM -1 (or internally non-load-bearing 10 instead of 11.5) were not due to aerated concrete plan blocks. If that is not aerated concrete, "I’ll eat a broom."

I may be terribly mistaken now, but I understood it to mean that the OP just wants to know if aerated concrete would be that bad...?

Well, after what you write, I no longer see any point in taking over this house. Why? You want everything different and in the end it's still a bad compromise. You sink the money anyway to the same amount as for new construction. Call the whole thing off, find yourself a plot of land and build new.

At the very first conversation, she threw a number between €180,000 and €200,000 into the room.
Now she wants to have the house appraised first. The house is located in Märkischer Kreis.
Based on my initial estimates, I came up with a renovation amount of €100,000–€120,000. Realistic?

Where exactly in Märkischer Kreis? There are very, very strong price differences; depending on the location, her expectations may be more or less justified.

Considering what you still have to invest to turn it into a multi-family house, I think the price range mentioned is too high in relation to what you will get. But it also depends on what budget you have and how urgently you want to live exactly there.