Build new or buy an old house?

That is purely a matter of negotiation. They definitely do not want to take that with them, but rather have it as bargaining chips. Depending on how interested I am in the house itself, I would take the stance: sorry, we don't like the kitchen anyway, there’s no time for a sauna, a pool just costs money. The fireplace is a nice-to-have. Then they will, depending on how many interested parties there are, put on a big act. Of course, it can also go badly for you. Heating costs are absolutely individual. Is the financing really so tight that you have to worry so much about that? The energy certificate doesn't say much at first. If you live there, you can still optimize the heating and insulate the basement ceiling a bit. Then the boxes of the roller shutters and other leaky spots. That already helps a lot.

This is purely a matter of negotiation. They definitely don’t want to take it with them, but rather have it as leverage.
Depending on how interested I am in the house itself, I would take the position: sorry, we don’t like the kitchen anyway, there’s no time for the sauna, the pool only costs money. The fireplace is a goodie.
Then they act tough, depending on how many interested parties there are.
Of course, things can also go badly for you.

Heating costs are absolutely individual. Is the financing so tight that you have to worry so much about this?
The energy certificate doesn’t tell you much at first.
If you live there, you can also optimize the heating and insulate the basement ceiling a bit. Then the boxes of the roller shutters and other leaky spots. That already helps a lot.

Yes, that’s what I assume as well. Nobody is going to take that stuff out anymore.
That would have been my approach too.

No, that’s fine.... I just like to calculate everything about 30 times.
We just want to get a feeling for whether we’ll be at x or y monthly.

Yes, those are good things you can do yourself gradually and that will help quickly or pay off.

For me, that would be an absolute no-brainer. The existing house is practically a gift at that price. Did I understand correctly that the location would even be more favorable for you than with the new build?

The existing house is a bargain at that price.

But there is still "plus X" in the amount of N.N. for various installations.

But there is still "plus X" in the amount of N.N. for various installations.

all three things would have to cost very little for one to actually want to take them over.... Or one could sell them afterwards, if desired..

If there are of course many interested parties, it may be that they bundle it and say: we only sell everything as a whole. A kitchen is practically never interesting as second-hand, because it never fits anywhere else. A sauna is also no good, same with a pool. For that, you practically get very little money in separate sales. Too bad, but nothing wrong with asking what they want for it... Otherwise: additional costs are very, very individual. I just can't give a reliable figure.