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

These are the two opinions between which we have to choose.

However, the first opinion applies to all old buildings (roughly older than 20 years). The value, however, does not drop to zero, the pressure is too high for that, and in the currently discussed property, the value of the house compared to the purchase price demand is already more than small today – the purchase price is only for the plot plus the basement. Above ground is given away. Or am I calculating wrong?

I would fully agree with the second opinion. Who nowadays wants to live in just one property until death? Those times are over, just like having a job with the first employer for life. A house is a consumer good, like a car, only with a lower depreciation.

Well, there is definitely a difference between comparing a solid brick house from 1980 with 42.5 cm thick walls and triple glazing and a prefabricated house with wooden walls and the standard from 1977. Just google for pollutants, carcinogenic substances and Okal Haus houses. When I read all that, I couldn’t live in such a house with children with a clear conscience. After all, you are not only responsible for your own health but especially for your children’s, who cannot do anything about living in a house with pollutants. The first mentioned house also has a normal energy consumption, while the prefabricated house shows catastrophic energy values.

Well, there is a difference between comparing a solid brick house from 1980 with 42.5 cm thick walls and triple glazing and a prefabricated house with wooden walls and the standard from 1977.

Great, you are comparing apples (unrenovated prefabricated house) with pears (partially renovated solid house). The latter will also be significantly more expensive to buy.
Apart from that, the energy consumption according to the energy certificate for the "apple" is not bad at all. In my experience, even better than most comparable (please do not take the partially renovated house into account) solid constructions. By the way, the 42.5 cm thick walls do not help the old building at all. I live in one myself and before renovation the energy consumption (built at the end of the 60s) was significantly higher than for the "apple".

Google for pollutants

Do not google. That is nonsense. Bring in an expert to the house. That’s the right way to do it. You have absolutely no idea what specific pollutants are or are not present in this particular house. Or would you avoid every old building because asbestos was used for everything back then?

The first-mentioned house also has a normal energy consumption while the prefabricated house has catastrophic energy values.

See above. Your comparison is flawed and the prefabricated house is bad, but by no means catastrophic.

So please argue a bit more based on facts and not on gut feeling.

And once again, the purchase price represents land + solid basement. The rest of the house the OP basically gets for free.

These are the two opinions between which we have to decide. And nothing is harder than that..

I don’t see what should be so difficult. You have been searching for more than five years, you have very specific wishes and everything is being fulfilled except that the house is not like new or solidly built (which is unrealistic with your budget and apparently doesn’t even come on the market) and the garden is not >1000 sqm. But the plot is not small and as I said, houses can be extended or rebuilt. There are currently even good subsidy conditions and presumably even more will come in the near future. If the indoor air analysis is okay, go for it. Or do you think that a dream property will suddenly come on the market cheaply in exactly your desired location and you will then have no competition?

We don’t have a dog (and if we did, he wouldn’t speak Swabian like his owner and mistress and therefore wouldn’t add any value)

Our dog doesn’t bark Swabian either, a Baden lady wouldn’t like that at all ;-)
But he is a first-class matchmaker for conversations with locals in the city, countryside, and on vacation, which you as a dogless newcomer would never get close to.

These are the two opinions between which we have to decide. And nothing is harder than that...

To me, nothing seems difficult about that,

Floor plans are top. There is sufficient space on the upper floor and ground floor for a study, children's rooms, etc. Only the wall between the and living room still needs to come down

I read that as basically a "sealed" engagement with the house already. Only

The house was built in timber frame construction in September 1976 by the company "Neue Heimat Baden-Württemberg"

still raises a small need for clarification. I am currently unsure whether NH (which only marketed but did not build the houses themselves) sourced houses like Neckermann does at Streif Haus or from Okal Haus or someone else. At least I can recognize an Okal Haus from pictures (views and floor plans) quite reliably, but possibly not a Streif Haus house or others. I already recommended the prefabricated house expert to you, who will also know more about older prefabricated houses. What is certain is, this is still a model from an older generation (as said, just a few years later the "prefab" houses were already on par with the "solid" houses in terms of value, and even ahead energetically – but apparently not this one). Find out about the wall construction. Whether insulation has sagged can be shown with a thermal imaging camera shot (see Energiesparkommissar), and mold infestation is noticeable in the indoor air measurement. In my posts about Kampa / Okal Haus with a similar question to yours, you can read why I rather advise against replacing the "chipboards". In the two threads by you will find the gutting of a "prefab" house described.

I therefore see hardly any significantly "unknown" factors remaining in your decision. The worst case is that the investment in the existing building would be uneconomical, and the plot would thus only offer a somewhat oversized basement. More likely, based on your previous description, you find the conditions manageable. Conclusion: the seller agrees to postpone the deal until after clarifying the wall construction (if necessary see building file), indoor air measurement and thermography, and you buy – or not, and you continue searching and live with the fact that I don’t believe in the failure of this further search :)

Attached are a few pictures..

Today we called three appraisers.
The first sounded incredibly knowledgeable. 73 years old, master carpenter. Expert since 1978. Knew exactly who the manufacturer Neues Wohnen is. Said they had production done in Saarland in Neunkirchen. Then he asked me how the house is plastered and where the roof slopes down etc. and based on the construction type knew exactly which materials were used and how. He definitely strongly recommends a pollutant measurement. He doesn’t know how renovation costs of €150,000 for a prefabricated house of this kind are supposed to come about. It would have to be made of gold. For example, triple-glazed windows are not allowed alone because of the 25% limit compared to the exterior wall, if I understood correctly. He generally criticized the insulation due to the risk of mold, without me being able to follow his explanations in detail. He also thinks it’s nonsense to pay the penalty if, after the current house purchase, you don’t insulate the roof accordingly - as determined by the chimney sweep. A lot is talked about these days, etc.
€700 for the pollutant measurement, €1700 including determination of building condition, consultation, and renovation cost estimation.

The second sounded less experienced. Recognized prefab house specialist, long-time senior construction manager in the prefab house industry etc.
He can’t measure pollutants; that is done by someone external with a two-month waiting time for results. Determination of building condition, consultation, and renovation cost estimation on site orally €120 per hour with an estimated 2-3 hours effort. Costs for written documentation would need to be clarified on site.

The third also did not know the manufacturer. Wooden houses are not his specialty either. He said you have to be extremely careful with prefabricated houses in timber frame construction from this period. Usually, you have to open the exterior wall from the inside, remove old mineral wool insulation etc., and rebuild the house accordingly. He says that considering the price you should definitely wait. In the next five years, an incredible number of old people will pass away and the supply will increase significantly again.
Determination of building condition, consultation, and renovation cost estimation on site orally depending on package €400 - €850, but it will be billed to the seller, from whom usually about 20-30% of purchase costs are negotiated for the buyer. I didn’t quite understand the billing.

PS: We are collecting experience values on renovation costs on the side... A colleague said this morning he paid nearly €60,000 two months ago for 2 windows 100 x 40 cm and a total 5-meter-wide floor-to-ceiling sliding door construction in wood-aluminum triple glazed going around the corner. Another paid nearly €12,000 for 7 plastic windows 100 x 60 cm about 12 months ago.