If you look at the bare numbers like that (as a layman), the whole thing looks very tempting.
How it really looks, you will see when you have a correctly prepared and unvarnished return calculation in your hands. If you roughly combine Bauexperte's rough cost calculation and your property valuation, the project/investment amounts to about €900K-€1 million including the land. Of that, you would then be able to contribute a little more than 20% equity in the form of land and the €50K in cash, with that you would have put in everything you have.
Of course, you can do that, but I wouldn't. If you decide to keep all three apartments completely, you should at least have the sale of one apartment or even the entire property in mind for emergencies. In case of doubt, you have to look at it quite bluntly from a business perspective and unemotionally. If you cannot let go at the right (wrong) moment because you cannot handle it emotionally, then that can definitely become a problem. Unfortunately, this is observed often enough, and it might actually threaten you, because from the €2.5K income you will definitely not be able to build up any or only a small buffer.
Best regards Dirk Grafe