Assessment after blast snake approximately 25% lower => extremely uncertain

The headline suggests that the combination of a "reputable appraiser" and a "significant value-price deviation" triggers your uncertainty. However, this is not unexpected: secondly, an appraisal is not synonymous with a price prediction, and firstly, in the market it is "value is what a fool pays for it." The more bidders foolishly compete for an offer, the more the value in terms of price will exceed the value in terms of the objective equivalent.

According to him, we were lucky that it was a "divorce house" and that we were the first to make a purchase offer.

You don’t buy divorce houses from the real estate agent, but from the lawyer

But can you still get a solidly built single-family house (built in 2014) nowadays with incidental construction costs and garage for 390 K€?

Yes, yes, that works out. Roughly 2000€/m² plus 50k incidental construction costs, 20k garage, 20k garden. Depending on the region and standard, more or less. Also note that with a new build without a property developer, you save the 12% incidental purchase costs on the 390,000€. That’s a lot. And you still have to renovate now, since the house was lived in for 6 years. I can therefore very well understand the low valuation. Purely based on the fundamental data, the house feels expensive. But if the location, equipment, garden, low renovation needs, etc., are right and you want to live there, then you can definitely pay the price. Better to move quickly to your desired place than to search for years and then have construction site stress. If it’s a divorce house, then the sellers try to sell at least without loss despite the cancellation compensation. Hence the quick agreement.

2,000 per sqm is steep, isn't it? Dr. xxx mentions something about 1,300. Well, not everything is perfect either. But we have been searching for a very long time.

Sprengnetter offers various valuation methods, some for offer price indication and others for purchase price indication after property collateralization. You can also switch between comparison, cost, and income approaches there. Different data will then simply come out. But these things do not replace a real appraisal.

2,000 per sqm is steep, isn't it? Dr. xxx mentions something about 1,300.

1,300 was definitely still from the DM times. Nowadays you don't get anything for that anymore, at most a shell. Often it's 2.xxx€

Then it's fine ... energy A+, cistern, underfloor heating, BUS, etc.