Ecologically built house with wooden facade - Features

I almost missed this part, sorry. According to the energy certificate, the energy consumption (heating oil EL) averaged over the last three years was about 13,000 kWh, of which about 2,300 kWh for hot water. Then apparently about 1,600 kWh from firewood for heating. So I assume 13,000 kWh x 1L/9.8kWh = about 1,320L heating oil per year. For a living area of 124m² / usable building area of 167m², that seems quite reasonable, doesn't it?

Numbers on paper (calculated) don’t really help. Is the house still occupied? Ask to see old bills/consumption and "feel" how warm it is in the "hut." Some already find 20°C warm...

In this range, a heat pump works almost like an immersion heater – energy goes 1:1 into the heating. The question is rather whether 35-40 degrees are sufficient. As a maximum. 50 is a deal-breaker for an (efficient) heat pump.

I feared that :(
I will look into what alternatives are available or follow up with the seller to see how it was all planned.
Thanks for the tip!

Numbers on paper (theoretically) don’t really help. Is the house still occupied? Get to see old bills/consumptions and "feel" how warm it is in the "hut". Some already find 20°C warm...

Yes, the house is still occupied, I will try to get insights into the old bills. We actually like it a bit warmer too, which is why the topic is particularly relevant. Unfortunately, during the viewing you don’t pay attention to such things, plus you rush through the house with a jacket and shoes on and accordingly get warm ;)

It is apparently a special house for its time and will therefore always fail the sieve in a purely technical comparison with the 2022 standard. A house built today would also fail the sieve of 2025. However, the living quality of today's houses is not better because of that.

Your focus is on flow temperature, wall structure, etc., and you are probably thinking through every worst case, but these exist for all types of construction or materials. Tiles, engineered parquet, masonry all have their disadvantages as well as advantages.
It seems to me that you are a bit anxious and engage too much with wall construction, etc. The house has now been standing for almost 30 years, and in 1996, good houses were already consistently built. Heating costs are especially determined by more or less sensible use by the occupants.
Apparently, someone gave special thought to materials and natural building substances when the house was built back then; I wouldn't even know what I could dislike about it, mediocre everyday houses are plentiful enough.
We built our new house similarly: a central stove that is currently crackling and in addition an IR heater or in your case radiators. That is exactly what I would have done as an alternative, and there are also really beautiful radiators or you can have them painted.
We deliberately did not want underfloor heating because for us it has disadvantages due to its inertia and the quasi exclusion of a wood stove; this was a fixed decision for us. Of course I have to calculate wood consumption too, but for us, it is not a matter of calculation but of desire.
Almost the entire house has solid wood floorboards, what worries you so much about that? In the bathrooms, they are also no problem.
If you spill a glass of milk, do you ask? Then you wipe it up just like anywhere else; the current owner has been living there for almost 30 years as well. If the floor is not treated at all, which would not be a problem, then you could wax or oil it. What happens if a jug falls onto the tiled floor? Both are broken.
I haven't seen the windows but why should they be replaced? Of course, you can retrofit external roller shutters; we did that.
Don't let yourself be frightened or guided by all these values. Would you prefer if the homeowner had used everything in PVC or painted it millimeters thick with some kind of paint?
Change your perspective; that was already a special house back then. By the way, we are currently building our wooden facade, even from spruce, and why should that be a problem? The 200mm insulation was already good for the standards of that time, and I have lived very well for 30 years with 200mm insulation.
I would ask myself if I could live well in it. Room layout, living feeling, location, orientation, etc.
As a forum participant, you can easily get the feeling that your own house is not worthy of being called such if it lacks this or that or certain values.
Again – don't let yourself be frightened by some of the topics here. What use is the record value of the heating system if the house is deadly boring, and it does not seem like you are talking about a rundown place. Such a house would probably appeal to us.

It is apparently a special house for its time and will always fall through the 2022 sieve in purely technical comparison. The house you build today will then just as surely fall through the 2025 sieve. However, the living quality of today’s houses is not better because of that. [...] Again – don’t be scared by some topics here. What good is the record heating value if the house is boring to death and it doesn’t seem like you’re talking about a dump. We would probably like such a house.

Wow, thank you very much for the detailed answer! I think with that you really hit the nail on the head, as a first-time homebuyer I’m probably just trying not to make any mistakes and getting too caught up in details. Actually, the layout and location fit quite well, everything is lovingly maintained, and one could move in right away without having to renovate much. Which house from the ’90s can you really say that about?

I will try to approach things a bit more optimistically, many things like heat protection or wood treatment can easily be adjusted even after moving in if you’re dissatisfied. The sellers haven’t decided on a buyer yet anyway, so maybe I’m worrying unnecessarily. So: thanks again for the help, I will definitely keep you posted on how things develop!

as a first-time homebuyer I probably just try not to make any mistakes and get too caught up in details

If someone built an "eco-house" back then, they took their time and effort to find the materials, usually even for more money. Just from this effort alone, in my opinion, the chances increase that you buy a "good" house. You can build everything in "good" and just as much in "bad", it is not a question of the material. I have already written it a couple of times. My first house was built in 1990, monolithic, about 20 cm insulation in the roof, double-pane pine windows, no shutters (I found them ugly back then—we wanted it bright & open). We corrected the mistake on the south side 10 years later with mounted external blinds, period! Floorboards in the living area are more of a luxury solution for me, since even prefinished parquet has only 4-x mm material thickness of the desired wood. I absolutely like floorboards, therefore now also in our new build, that is something for eternity, if you want. Someone moved directly into my house when it was sold, what a luxury. If that works like that, I would probably do it too and then live in it first and see what I want to change. Heating costs are, as mentioned, often the problem of the respective users, and I like the combination of wood/ radiators anyway.