To give you a small point of reference: At a bank well known to me professionally - as already wrote - a simplified income-expense calculation is created using flat rates and formulas. This allows the decision-maker to quickly estimate whether it makes sense to delve deeper into the case.
A (simplified!) calculation would then be based on the numbers you mentioned (1,500€ net, 80 sqm desired living space, older building)
Additional costs 240€ (3€/sqm for an older building, which then covers electricity, property tax, water, garbage collection, etc. etc. etc.)
Living expenses 700€ (minimum assumption, even if you spend less)
Car 200€ (minimum assumption)
Property insurance 50€ (minimum assumption)
Installment burdens 150€ (as you indicated from earlier studies and no, something like this cannot/should not be refinanced with a building loan)
This means that of your 1,500€ net, 1,340€ are already gone at this point and that’s before any building loan installments, life insurance, savings rates, etc.
Sorry to say it so clearly, but with 1,500€ your creditworthiness is significantly below average - especially since the residential real estate credit directive, but also even before that.