House purchase single-family house built in 77 how to assess?

We would initially want to leave the insulation of the house wall, water pipes, and electrical system as is for 5-10 years..

Hm.. regarding that, I would look up the topic of renovation and the EU program "fit for 55" again. It doesn't really affect the water pipes and electrical system, but it does concern the insulation. However, I don't know what values the house currently has, but I was quite surprised by the ambitious goals - which start to take effect as early as 2030...

The new partner of the seller - who probably has a lot to say - already said that he is not willing to go down even one single cent

I would say that too, if the fish has already taken the bait and is only wriggling on the hook... That reminds me of another user who wanted a particular house so badly that in the end he was willing to accept almost anything and even paid more than he wanted.

Tonight we are thinking again of canceling or negotiating down from €365,000 to €315,000... which is unrealistic and tantamount to a cancellation

Of course, it is a risk to name a lower price and wait and hope that they will eventually agree and no one else will bid. On the other hand, so far there doesn’t seem to be anyone willing to pay the asked price, and if they want to sell, they have to go down sooner or later or wait indefinitely for day X until maybe the person who will pay the price comes along.

We recently demolished a house like the one shown on page 11 of this thread because the general renovation would have been unrealistically expensive and things like basement waterproofing would have been hardly feasible. The structural integrity also would not have supported our plans to add another floor. In the end, there were too many compromises for still way too much money, which is why the wrecking ball came. In our prefabricated house from the 1970s, we were particularly bothered by the following things: - harmful substances such as Xyladecor in the roof structure, asbestos in the facade, in tile adhesive, in the adhesive for the floors, in the vinyl, as well as formaldehyde in the OSB boards, etc. - the typical "old people smell" – which is actually caused by so-called chloranisoles and does not always completely disappear even after extensive renovation - basement waterproofing with bitumen thick coating had actually already reached its end of life - conservative floor plan - and much more. Personally, I would not invest in a prefabricated house (at market prices) where the aforementioned weaknesses have not already been repaired by the previous owners. We had bought the house within the family and only for that reason.

We bought a prefab house from 1994. Back then (2013) we made an offer that was 20% below the seller's asking price. We were very interested in the house. The seller categorically refused and was even outraged. Four months later, we reminded him ("You don't want the house to stand empty over the winter...") Eventually, we agreed on a price close to our offer. Yes, 2013 was a completely different market, I know. But I am glad that back then we relaxed a bit and took the risk of not getting the house.
So I would also advise a lower offer, one that you feel good about. And then stay calm.