Construction cost breakdown plausible?

Well, I'm still not entirely sure about the air heating. Some are familiar with a geothermal system combined with underfloor heating. This can also be used to cool the house in summer. However, I have already read that the systems from W are not supposed to be particularly good.

The larger prefabricated house builders have all started installing these air heaters as standard (or at least so that the already inflated price looks a bit lower) and this air heater, as far as I know, always comes from the same manufacturer. You save a lot of pipes, the entire underfloor heating, etc. in the initial investment, and afterwards you have a KFW55 house on paper which has significantly higher consumption costs than a 2014 Energy Saving Ordinance building constructed to minimum standards. Incidentally, the ventilation system no longer just ventilates, but is supposed to be a heating system as well. Especially in winter, you want to rather reduce the air exchange rate because otherwise the humidity drops too much, but with such an air heater you have to increase it so that the hot air (a very poor heat carrier compared to water) at least generates a bit of coziness.

Does the house have an installation level? Some prefabricated houses do not have one, and you have to consider whether you want it that way.

That also probably costs extra at W, but Ghostraider can certainly take another look at his offer. If it is not explicitly stated, then it is not included.

I have not read anything about an installation level so far. What exactly is that and what is it used for?

For the floor coverings (I assume you want tiles in the basement as well), that would be about 40.-/m². I find that quite low, even if you lay the tiles yourselves. Nice tiles alone can easily exceed this budget, and there's also adhesive, 10-15% waste, grout, silicone... Count on at least 15,000, more likely 20,000.-€.