Very appealing! Apart from the fact that the renovation costs can hardly be estimated here, they also depend on what you plan to do with it. Live in it year-round? Use it as a vacation home and rent it out?
1. Is the monument protection actually in place?
2. Energetic condition is a disaster - before and after renovation
3. Costs: To properly renovate the mill, I estimate around €500-750k. However, it could also be more.
The thing definitely has charm and, if properly renovated, will certainly be a real gem with history.
I cannot give concrete estimates due to lack of expertise, but in my opinion, it is not a property where you invest tens of thousands of euros to make it livable. So it won't be an affordable home. Rather, it is something for a liquid enthusiast. Emotionally, I would align myself with the aforementioned figures, which lie somewhere between 500k and 1 million.