Is a house for a total volume of 400k still possible at all?

The first two pictures seem to me to be a glaze on a white/light background, with the first one possibly not yet dry. The third picture, of course, looks like that only when viewed up close; from the middle of the room it looks like a paint without grain. When you are close, you can see a 0.5mm grain. You do have some direction when painting, unless you spray, and this can usually be seen. The question would be why you are even considering a brush-on plaster if such a "grain" bothers you. Maybe a more or less normal wall paint would be the better choice? It is like on the floor. A single-color tile shows more unevenness or dirt than one with a texture or grain. Or with a wooden floor, one without knots/lively.

Well, my wife doesn't like it.
So far, from the various threads here, I have gathered that Q2 either has to be filled and sanded multiple times before it can be painted directly. Or wallpapered with thick painter's fleece and then painted (possibly also requiring additional filling and sanding here).
Or it could be treated with already pre-colored paint/roll plaster. I had the naive hope that there is a very fine version of this so that you could get a smooth surface without streaks.
Because I lack the time for the other options (not necessarily daily time, but rather the time between acceptance and moving in), the skill to do it myself, and the money to have it done.
That also establishes the connection to the topic. I also have to stay under 400,000 EUR...

Would the question be whether you can do it without painter's fleece, using only super fine brush plaster.

Don't you have a small wall where you can try it out? Somewhere in the dressing room or cloakroom? I also once tried 's quartz sand in color variant: back in 2000, it was like this: the brushes (I used a whisk brush) were clearly visible.

Tolentino, my version looks like the third picture from the top. We then painted it with a slight tint. We didn’t want to leave it pure white. You have to understand, you need a certain texture. Because instead of long filling and sanding, we want to mask the irregularities of the walls, which is achieved through the sandy texture. The smoother it is, the more you will see everything. There is no free lunch, 400 thousand minus or smooth walls. Choose.

That’s the problem when you have kids’ pop walls, that you will see every tiny unevenness. Now I don’t really know the difference between this Q2 and 3 but I also think that there will be significant differences in the respective execution. I think too much is done with such values and always at the highest Q-standard, then I see some walls and think I could have spared myself this expensive standard. As describes, image 3 will be, in my opinion, a very good condition that you will perceive as cleanly painted but just not photographed from 50cm distance. Currently, for example, we have fleece everywhere and paint on it, that’s just as okay. Personally, I especially like the textured paint because it tolerates or covers unevenness, also for me big cost differences would be a criterion, because better/more expensive is always possible. Maybe you should go somewhere where you can see that, usually also in public buildings or your painter could give you half a liter to try out. Maybe your wife is worrying too much, which could be resolved by a visit, because the longer you look, the bigger the grains of sand become. We just had that with another topic as well.

For example, I count 8 outdoor sockets and 7 outdoor fireplaces more as nice to have.
Makes sense to do immediately? Yes!
Should every home builder do that? No, not necessarily.
And your argument that you then just take out a 10,000 euro bigger loan exactly plays into that:
Houses are getting more expensive for that reason as well. People convince themselves that every additional euro is totally reasonable.

I also thought of the examples at the point "I wouldn't necessarily order that now." But that is a different argument than originally. The argument was once "in the past, many things were done later and therefore it was cheaper." Which first of all usually only redistributes the total sum differently and second often is not sensible. That's why, for example, I also include the outdoor facilities in the calculation. You have to pay for them anyway, and living on a construction site for years is also not sensible regarding quality of life.

The thread question is whether it is still possible at all to build a house under 400k.

I think that has long been settled. Including incidental construction costs and outdoor facilities, it's hardly possible anymore and usually involves some own contribution. But then you don't have to discuss a few outdoor sockets anymore; the big cost blocks lie elsewhere.