Renovation costs for a solid house built in 1985

I consider the calculation to be realistic.
With the floors, there is the most room for maneuver, I would say, it depends somewhat on the condition of the subfloor after removing the old floors. I would have tiles laid in the kitchen, though, that already costs a bit for 20m².
Bathroom and guest WC should also be feasible with non-exclusive fittings. If you manage it so that you buy the fittings yourselves and only pay for the installation, even easier.
The fresh water piping could also be redone on this occasion, but of course it depends on whether you have quite hard water in the area.
Kitchen is to remain? Otherwise, that would of course be on top.

The kitchen would be paid for by mother-in-law, budget approx. 15-20k.

After it looked relatively "smooth" at the beginning, the community of heirs is now acting difficult. Our contact person was so far the older brother, always nice contact and very informative. Yesterday his sister called me and said rather arrogantly, "so here in Düsseldorf and surroundings the house would fetch at least a million, she intends to market the house herself." Since my peace of mind does not depend on a house, I reacted relatively coldly and said that we are more than 200km away from Düsseldorf and that we are not talking about Meerbusch here. Since then there has been no message and I almost assume that we will soon find the internet listing on Immoscout for well over 500k (which it simply is not worth and where the market here is also very, very thin in terms of buyers).

Thanks for the helpful input here, if there is anything new I would get back in touch.

The problem is that with inheritance communities at least one person always causes trouble. Someone thinks they can make a huge profit and forgets that they only get a share. You could be taken advantage of in the process. Then there is exaggerated speculation.

I hope it will turn out well. Be patient. The problem is that it might be sold through a real estate agent, so make sure you get your negotiation in writing.

Otherwise, I don’t see any further exorbitant costs now. Certainly, the heating system could be replaced... Eich is still installing a buffer :)

And keep us informed about what happened :)