Garage or hall for 5 cars - How do I approach this?

I did not claim that a rental garage is unsafe, but that it can be terminated at any time is maybe unlikely, but always possible. And if you also store some car parts, that is already a reason to move. And I see no reason why a car would rust faster in a metal garage than in a brick one. Provided the metal garage is sealed and well ventilated.

I am now at the point where I will first continue looking in the stock for a small hall or inexpensive garages. It’s also less effort than such a construction project. I have gained important insights through you and thank you very much for now!

Oh, one more thing I'm curious about: If someone only builds garages on a property, does it have to be designated as building land for that? And if not, then what is it designated as?

Must be building land. There are exceptions for so-called privileged projects (agricultural, special industry, etc.), but that does not apply to you.

Best regards
Dirk Grafe