Financially a safe bet. The option to change the repayment rate also gives you enough flexibility for the time with children.
The question I have: why build already? You apparently live rent-free, are young, and have all the freedoms. If I were you, I might still enjoy the freedom a bit more (travel / party / sports / whatever) and meanwhile save up significantly more equity. You build when the family planning is concrete or even completed, then you really know for how many people you are building. No one wants to hear it, but the number of involuntarily childless couples is simply shocking; even in my limited circle of acquaintances, it is an incredible issue that you normally don’t hear about. On the other hand, twins can also come, and then it becomes 3 or more. Or is the current living situation not ideal, and you want something bigger anyway?