Concrete house, concrete foundation, and concrete columns - costs?

Hello, I have a few small questions for the experts, even if it is probably just a figment of my imagination, I would still be insanely interested in several details about it.

I would like to build a house and am playing around with a few ideas, maybe you can help me a little?

A small plot of about 150 - 200 sqm is intended, and a small concrete house of about 80 sqm.

I imagined it like this: concrete foundation about 80 sqm, then 4 concrete walls placed on it where, logically, it is already planned where windows and doors will go, the ceiling also simply a concrete slab overhanging all around, inside concrete columns as support for the ceiling, which I cannot do myself and therefore would have to buy somewhere.

On the outside I would apply complete thermal insulation and plaster it with rough plaster, on the roof a complete solar system, inside the walls clad with gypsum, real wood parquet on the floor and everything else that belongs in a house.

Do you have an idea what the concrete foundation + 4 walls and ceiling and the concrete columns could cost?

Is this meant as a holiday home?

no, rather as a 2-person household

Structural engineer? Architect? Building application? Milling pipes in concrete including house connections? Energy Saving Ordinance? Roof? ...

Structural engineer, architect, do I need something like that? I don’t need to mill pipes into the concrete because that comes behind the drywall; I would have the people who also pour the foundation handle the house connections since that’s something I can’t do myself either. I didn’t know you needed the energy saving regulation and that I have to pay attention to something there, and the roof is a bit of a matter—I actually wanted to waterproof the concrete slab and use it as a flat roof like you know from office buildings, where there are just pebbles on the roof; in my case, the solar system is supposed to cover the entire roof area anyway.

I find your questions about this great and I think it’s nice that you don’t all just laugh at me here; it will probably only remain a pipe dream and never happen, but I know myself that if I don’t inform myself about this now and talk to others about it, I won’t get these things out of my head for weeks.

I know it’s probably a lot of work, but I can do almost everything myself (with friends and acquaintances) except the shell of the house—that is, the foundation, the 4 walls, and the ceiling/roof and the house connection; we have plumbers, electricians, drywallers, roofers, and window installers in the family and close relatives, so that’s why I’m also so keenly interested in what roughly such a shell would cost.

Definitely more than if you build the shell structure entirely with traditional masonry. Why does it have to be concrete specifically? For the base slab and ceiling, that is clear, but walls made of concrete only make sense if they are particularly heavily loaded. And concrete columns like in industrial and commercial areas are used to remain very flexible in the floor plan.