Building from 50 - experiences with short-term loans?

Life is often unpredictable. You do not have to justify yourself for that.

Since you have relatively little equity, I assume there has been no real estate ownership so far?
You have "paid off" a child and probably have not built up any real estate ownership.
Now you want to achieve three times that (children + house).

Unfortunately, building in rural areas is no longer as inexpensive.
An extremely cheap existing property might be possible with extreme effort and a lot of personal contribution. But that also conflicts with the desire for a family.

Is there a chance for significant salary increases? Further training and job change/promotion?

Price increases of up to 10 percent per year are currently occurring

With regional companies and a tightened wallet, you might still manage to stay under 2,000 euros per square meter plus additional construction costs (in 4-5 years)

And even if so. Calculate the monthly burden you would have to finish by age 67.

You know what children cost and by the time you retire they still won’t be financially independent
So build reserves
Yes, paying off is terrible in connection with your own child

Only one thing is certain: the pension in 20 years will be lower than it is today

Ok, I have only looked at catalogs so far. Turnkey houses. For example, Danwood from Poland (?). They offer turnkey 130 for 160,000. Plus own wishes I thought 200,000 is realistic?

Definitely not! From €300k you’re in. And then the outside still isn’t completely finished and inside without a kitchen!

Turnkey is not move-in ready. There's still quite a bit missing. And the ancillary construction costs are added on top. You also don't have any outdoor facilities yet. There are a few examples here that managed to build cheaply. But that was also a few years ago. I don't think anyone ended up at 200k all-in.

...and how many times do I have to ask, please...make a list of all the costs you believe you have including [Baunebenkosten]. If something is missing, we'll add it. Then you'll quickly realize that it goes over 300k€.