Is financing over 40 years possible/sensible?

What are the remaining costs? Muster bathroom?

Sorry if I find your cost estimate strange, but you can't even calculate your own loan. Why do you then think you can reasonably manage your construction estimate!?

If the property in the Hamburg metropolitan area really only costs 150 euros per sqm as you stated, there must be a catch, and a big one at that, or your definition of [Speckgürtel] is very generously interpreted.

Once again about the term of 40 years. The longer the term, the lower the repayment, the higher the interest. Therefore, it is good to repay as quickly as possible (short term). The ideal extreme case is not to take out a loan at all (0 euros interest), the other extreme case is to never repay (then you pay interest for life). Everything in between can be calculated quite well by yourself ([loan calculator on the internet], Excel, pencil/paper, ...).