Thank you very much for your answers and hints.
I am not looking for a bigger house at all; 150 sqm of living space and a small garden would be absolutely fine for us. Unfortunately, there are no properties that match this scenario and are priced between 400-500k. Otherwise, that would be my first choice to set up a "secure" financing. The houses offered between 400-500k are in severe (!) need of renovation, and I simply do not feel up to that.
I will read up more closely on the topic of oil heating in BW, thanks for pointing it out... As for the other (renovation) topics, I must admit that I have hardly any knowledge and will therefore soon visit the property with an expert. Hopefully, it will then become clear what will be coming in the next few years. One note: The freshly determined energy efficiency class is "B". Does this allow any conclusions about the heating or renovation needs?
Regarding financing: If I have to borrow 660,000 EUR and the property is only worth 590,000 EUR, then this is about a 110% financing. What will the average interest rate be here roughly? Do many banks do this, or is it rather "a handful" (credit score is good)?
Best regards