Just because a private loan does not charge interest, it does not mean at all that Janosch does not have to repay it. Sure, there are agreements like, "You pay what you can" or "You pay starting from the 11th year" or "You only pay 50%" etc.
But it just comes on top. Besides, the bank will want to know where the money for the property is coming from if you are not listed in the land register.
Since you didn't say anything about the size and equipment, I assume a medium standard. So air-to-water heat pump, controlled residential ventilation, underfloor heating, fireplace, electric shutters, colored plastic windows, etc., without a basement.
Including ancillary costs, it will be almost impossible in a usual size of 150-170 sqm.
Especially since you, as a layman, already assess the property as having a "slight slope," it will probably be something around 1-2m or more in reality. Compensating for that can easily "eat up" 30k€.
But anyway, these are all just assumptions meant to get you thinking about the total construction cost.