Buy an old building to rent out?

The property is probably around 80-100€/sqm as is usual in the area I don't know exactly at the moment

again you also have a duty of care as a landlord. There are also rental agreements with the obligation that the tenant renovates the place. then you are off the hook. only then is the rent so low that it is not worthwhile

The plot of land is probably around 80-100€/sqm as is usual in the area

Well, then let's say the plot of land is worth 64,000€, leaving 100,000€ for the ruin itself.

I would then set the limit at 150k renovation costs. For 250k, a renovated old building is probably still worthwhile. For maybe 100k more, there would be a nice new building.

The maximum amount I would calculate again based on the expected rents and all that stuff.

The method of creating capital through a rented property is well proven. It is good to be liquid enough to carry out repairs and renovations when they are due.

Renting out a building with a renovation backlog only makes sense if you do not want to keep this property - for example, because of a new construction or selling a building plot.

Anyone who knows about renovation and has a functioning infrastructure of craftsmen and helpers can make some bargains. You do not seem to be an expert, and that is your risk. What advice could anyone give you other than to hire an expert?

If you rent out, everything must work or be repaired in case of a defect.