Our first house - your advice is needed!

but I understood it as the OP just wanting to know if aerated concrete would be that bad...?!

On the contrary - it would very likely mean that the building could be brought up to the current energy saving ordinance standards with minimal effort. I always find it pleasant when you can find indicators about the wall structure of a property you’re interested in buying just from the floor plan, even without material samples. Being able to answer the question "do I have to stick insulation stuff on it?" with a confident no already means quite a few bucks when you want to estimate your effort.

Where exactly in Märkischer Kreis? There are very, very strong price differences, depending on the location your desired expectation can be more or less justified.

Lüdenscheid. You are among your own here. I even seem to remember seeing on a relevant real estate website that a house on the street was listed for almost twice the price. Well. Let's see how it goes.

I would first ask myself - and answer as honestly as possible - whether I really want to live in the house with the parents. I really love my old folks and we have a great relationship, but living together? Probably not... the expectations are just too different. Let's say I don't meet the standards in terms of order (interpreted very generally).
And conflicts seem to be programmed here as well, as can already be read from the previous posts.
Only when this question is clarified can one think further: what budget is available, and can one manage with it? Nothing has been written about that yet.
A thorough renovation can quickly consume 1000€/m².

whether I really want to live in the house with the parents.

Not to forget that in a marriage, the parents of one are at the same time the parents-in-law of the other.

I definitely thank you all for your opinions, suggestions, and ideas. The topic can be closed to this extent, as Father will likely be able to call himself unemployed in the very near future. Thus, the topic is off the table. Thanks again to everyone.

So the matter is off the table.

That was your guardian angel, believe me.