I would recommend Frankfurt, it's a pity Munich is not on your list, I go often there and to Frankfurt for business. Both cities are great with lots of things to see/do. I am also thinking about buying apartment in Munich
https://tranio.com/germany/bavaria/munich/apartments/, haven’t yet ask my bank about mortgage. I heard that German banks offer good deals on property mortgages. Anyway, this is another story
have a great trip in Europe!