I have been doing a lot of calculations related to all this apartment buying and selling. Financially we are looking for reduced total cost of living so it makes sense to maximize the mortgage time to get the lowest possible monthly payment. Of course a long payment period will most likely bring us into an era where the current super low interest rates are history. So in this sense it would make more sense to try and pay the mortgage off as soon as possible.
Another option to make our payment plan more future proof is paying more up front. The difference between our ideal and acceptable buy- and sell price are both 10000€. So between everything going as planned and having to take the worst offer is 20000€.
If we would take that 20000€ difference for 35 year payment time it would mean about 50€ per month difference at current rates. So if we pay everything now we could effectively start investing that extra 50€ monthly. Over the 35 years loan period with a conservative return on such investment (5% pa) that would make about 60000€ of investments at the end. Not bad, tripling our money. But what if, instead we would just invest the 20000€ right away with the same expected return of investment? That would be 110000€! I think the choice is obvious. Now let's just hope we end up with that extra 20k€.