What costs will we face?

One more "stupid" question. We want to pay for the plot in cash. If we could spend €450,000 now for the house (turnkey) + incidental costs. Approximately how much should the house with the foundation slab cost? I really have no idea how much one should roughly calculate for the incidental costs.

The real estate transfer tax only has to be paid for the plot of land, or is there another charge once the house is built?

I would be extremely surprised, but I have not read up on Austrian law on this. In Germany, nothing additional comes on top.

The connection fees, if I saw correctly, can be obtained from the municipality.

That’s how it is in Germany. Call the local building authority or citizen’s office.

Do you still need a Telekom connection nowadays? With us, everything already runs via stream/WiFi...

You don’t "need" one if the mobile network coverage is good enough and you don’t use time-critical applications. For us, the purely mobile solution would be significantly more expensive and at the same time worse.

Would you, for example, build a garage immediately (have it built)? Or try to keep the monthly payments relatively low and save up and pay cash for the garage?

It’s a question of how important the garage is to you. Cars don’t suffer noticeably during normal ownership periods if they stand outside; for snow protection and to avoid fogging, a carport and/or an auxiliary heater is sufficient; for a company car, I wouldn’t build a garage anyway. I have always considered vehicle storage separately, whether for the terraced house earlier (double garage from the prefab garage provider) or now (double carport, which also protects the motorhome).

Are solid houses still "more valuable" than prefabricated houses nowadays?

No. Location, maintenance condition, and workmanship are far more decisive factors.

One more "stupid" question. We want to pay for the land in cash. If we could spend 450,000€ for house (turnkey) + additional costs. How much could the house with foundation slab approximately cost?

The question is not stupid, because what I get for a given budget is a sensible approach. However, a general answer is difficult, as there are individual parameters that are better clarified individually – such as local connection fees, necessary or desired work on the outdoor area. Roughly estimated, I would assume 375,000€ for house and foundation slab. But that is "Hornberger shooting."

I really have no idea how much to calculate roughly for additional costs.

You’ll get that soon. Pick up the phone, call the tax office and the municipality, and speak with various people there (building authority, water authority, ...)

So you can really skip the coaxial cables today. Years ago, I also installed them in every room during construction. Nobody needs them anymore today. Everything goes over WLAN.

I spend more than 90,- per month on TV+Internet! A satellite dish is still justified in new buildings.

One more "stupid" question. We want to pay the land in cash. If we could spend €450,000 for the house (turnkey) + ancillary costs, how much should the house with the foundation slab roughly cost?

There are no stupid questions…
Roughly €100,000 for all ancillary costs (construction incidental costs, garden landscaping, gardener/landscaper, garage/carport), the house may roughly cost €350,000. Roughly!

Would you, for example, build (have built) the garage immediately?

Maybe I don’t need one. Maybe I already have one… maybe I have more money than you?
Answer: our opinions don’t matter here.

Or try to keep the monthly installments relatively low and save up for the garage and pay in cash?

And what if I inherit next year? Or if you don’t have any savings rate at all? That kind of thing has to be decided individually for oneself.

Are solid houses still "more valuable" than prefab houses nowadays?

Such questions can be saved if you are sitting in front of a forum asking this, although you can also read the countless posts about it in the forum.

I really have no idea how much one has to calculate roughly for the ancillary costs

Then just read in the forum ;)

Ps
I would not do without cables for TV or satellite. It’s like heating. Why have heating if a) it’s warm anyway in summer and b) there are hot water bottles.
You need the provider for WLAN, streaming is not a miracle of the world: through the "I only watch what I want" a lot of general education is lost in the individual or a source to absorb unnecessary information for oneself is missing ;)