Realistic purchase of land and new construction of a single-family house & how to finance it?

500k with a good income is steep. If your wife goes back to part-time work, it looks better. In any case, I would consult an intermediary. Whether using a [Bausparvertrag] here makes sense, I doubt. Definitely fix the interest rate for 15 to even 20 years.

Definitely fix the interest rate for 15 up to even 20 years

Whether this is worthwhile should be calculated individually by each person.

15 years for the biggest chunk would be the minimum. As I said, a good mix would be good.
Regarding the initial purchase of the land, I have read that there should be no problem if it is registered as a subordinate land charge (otherwise I might have an interest/bank problem later with the main loan).
Is that only possible with building savings contracts?

Assuming: 235K + 5% notary & tax = 247K
minus 145K cash funds
minus 40K "loan" from family
remaining loan amount 62K - should somehow be possible to register it as subordinate.
We wouldn't need the biggest chunk for the house construction right away, but only in 6-12 months...
In our Wohnriester there are another 25K. But I don’t know yet how I should/can use that.

Sure, I have to go to the experts and get advice, I just want to have a rough idea beforehand...
Furthermore, the land purchase is not 100% certain yet, because there are many (!!!) interested parties (but we know the seller... only who knows...)… how do they all manage that...??