Construction financing for land and all incurred costs, OK?

That's why I asked about the rent. If it is around 690,- EUR warm, then possibly cold rent is 500-550,- EUR. That means you have to set aside at least the same amount again for financing the installment. Rather, the installment will be around 1100-1200,- EUR. On top of that, there will likely be higher additional costs for the building and insurance, etc. I’m not even talking about forming a reserve for the building yet.

Without knowing what you have done with your income so far, I recommend you start setting aside 1300-1500,- EUR per month from now on and then see how the month goes and whether you can manage with it. If yes, then try again next month :):). If one of you starts to "whine", then rather do not finance. But if you can restrict yourselves, then it should be no problem in terms of income.

300 euros for additional costs is even nicely calculated... we calculate with 400 euros. Property tax, electricity, water, garbage, building insurance...

I also consider the financing very risky. With 4,000 net, 1,000 for living expenses per month should be possible for food, clothing, etc. If you subtract another 500 euros for VBL like mobile phone, life insurance, 2,500 euros remain for the house including additional costs.
500 euros of that as a reserve in case something breaks, you’re well off with 2,000 euros... you must not forget that you are already over 40.

If you were 30 now, the world would look different.
And what you have done with the money before is nobody’s business. However, if you have lived a lavish lifestyle, e.g. always traveling a lot, luxury clothing, etc., then the downsizing will be difficult for you... as I said, but no one’s business.

People, the effort is not worth it. The Lord has withdrawn... maybe this also explains (Kritikunfähigkeit und Rückzug) the meager substance ;)