So I think 350k is already a lot in this constellation. Let's assume the woman will only work part-time after parental leave. Then they have about 3.7k including child benefit per month (maybe a bit more with a change in tax class). I find a 1250 rate OK for 350k. But that would mean 2.5k for living expenses per month. That's doable. However, I would only feel comfortable with that if there are still 13th monthly salaries or bonuses (of course these won’t be paid this year, but in the long term there is at least the prospect of slightly higher income). But it’s a matter of taste. Maybe they are also very frugal if most of the equity capital was really saved up and only 50k was inherited.
Furthermore, lower construction costs of 50k are certainly possible. Then only 300 would have to be financed and the monthly rate could perhaps be lowered a bit.