How do I finance and build correctly? Solid house or prefabricated house?

Let's just assume that it is not an economically strong region / rather rural - otherwise the parents would not offer 1,600 sqm for €80,000 for apartment renovation. Then it will be a small off-the-shelf house = €250,000 plus own contributions. Thus, about €250,000 needs to be financed with reasonable use of equity. Relatively young OP, thus 2% repayment with rather longer financing term. That results in a comfortable 4% annuity of €833 monthly and is therefore well financeable with his income (no liabilities assumed). This gives the OP a first, realistic assessment. If it gets serious with the girlfriend and children are planned, he can also build bigger if the girlfriend is also jointly liable.

At least it would be important to have a cost figure as a basis or alternatively a desired interest and repayment rate, where the cost figure can be calculated.

Also important is the knowledge about the investment location due to the inclusion of regional banks.