Is building a house affordable for us? What is realistic?

so no, with your conditions it will be a very tight fit.. I would leave it with the new construction and stick to the "dream"

Exactly! A quick rough calculation of the years worked compared to the saved amount + property including the statement that everything was self-saved shows: something is not right here! But it was still nice to read :p

It would probably be too exhausting for me to have to take care of everything in the "own home" myself...

better to casually spend 1500€ for rent including utilities and have no worries...

So, kidding aside!

With the given conditions, I would plan a nice house there (PS. I have also experienced in my circle of acquaintances, saved a lot of money, then built a dream house for 2 people, as no children were planned)

I would plan the house to be big enough (nice large rooms) but not too big (for when you get old)
Also preferably a bungalow, so nicely age-appropriate.

Then also as a passive house, or with geothermal/air heat and with a photovoltaic system, storage. So you can live almost self-sufficiently and be independent from rising electricity prices.

The difference that is missing, I would then finance, as a full repayer in 25 years with a 1200€ rate, you could certainly finance over 300,000€.

Thus you would have just under 600,000€ as a budget for the house, more than enough. But I think you should also manage here with 500,000€. (without basement)

So calmly start looking for developers, house building companies etc.

There are also nice exhibitions, for example in Frechen near Cologne: FertigHausWelt Köln

There are also nice exhibitions, for example in Frechen near Cologne: FertigHausWelt Köln

And how is she supposed to get there? By car?
She does have 5 brand-new luxury cars parked outside (all paid for in cash, of course). But driving is kind of unsafe. Just recently heard about a car accident in the media. And there was also an unsafe driver in the family once.
What do you think? Should she really drive one of the cars from the family fleet? Or better sell the cars?!

I find it extremely difficult to spend money.

Then you should consider why 170 sqm are necessary for two people. That is already a generous area and, of course, it costs accordingly. Is there any idea how this space is supposed to be used? If the given framework conditions are correct, you can of course also build yourself a palace. But if you actually don’t want to spend that much money.... It is probably possible to go a size smaller and thereby also cheaper. It’s a pity when you have actually made it and left behind poor conditions and still cannot relax when it comes to money.

So under these conditions, I wouldn't build. Or at least much smaller. Why do you want to own property? You don't plan to have children, earn a lot of money, live nicely (I assume). I would work much less and enjoy life. Travel, be independent. With a house "attached," you are no longer that. Property is a security for the family for me; if I were alone, I would spend my money differently. You don't take anything with you, and I think experiences are worth more than anything material.