Rule of thumb financing rate

But later on as the sole breadwinner and with 2 children, is a 700 euro installment rate okay then?

No one can tell you that because no one knows your expenses. Therefore, the tip to keep a household budget book is the only correct one. For me, that would be a no-go with that net income. Because after subtracting the installment and additional costs, there are only 1300 € left (with child benefit). If BeHaElJa says it would be okay for him, then I want to believe him. But: even a 10-year-old car costs insurance, tax, gasoline and maintenance, and after another 10 years maybe a new one will be needed. Also, the vacation at the North Sea (which is significantly more expensive with 2 children than just the two of you) needs to be paid for. And if you have to put 500 € on the table for childcare, then there isn’t much left for food, insurance, telephone/internet, etc., let alone for any hobby, not to mention the money that should be there in case something breaks or needs to be newly purchased/wanted.

The other question that arises for me is, of course, whether you will be able to afford a new build at all with your desired installment rate of 700 € and your equity (that was 15k, right?).

Regarding equity, I will see over the next period what else I can mobilize!

The other question that arises for me is, of course, whether with your desired rate of €700 and your equity (that was €15k, right?) you will even be able to afford a new build.

That probably depends on where the OP lives and what he wants to afford. If he has the plot (virtually) for free and simply puts the simplest catalog house without extras with a lot of own work on it for €150k including incidentals, then that won't be a problem. If he lives within a 100 km radius of Munich, then he can't even finance the plot with that. Nevertheless, I would consider a part-time job for the girlfriend on such a tight budget. A few hundred euros in that net range already make a big difference.