Is this financing feasible?

When is it supposed to start? Do you want to get married beforehand? With the difference in contributed equity and a 50:50 entry in the land register, you have to be a bit careful regarding gift tax.

I assume you are building the 130m² living area on a flat plot without a basement. If that is not the case, you will not be able to cover the exterior facilities (e.g., retaining walls for uneven terrain) or additional construction costs (earthworks due to basement excavation). A tip: get an alternative offer from a local solid house general contractor. I also planned for a long time with the companies you mentioned and then switched to solid construction and am very satisfied with the final result.

If I may add my personal opinion,,, I would buy a building plot without a construction obligation. Then keep saving until you have €100k in equity, then start and the children are involved from the beginning and there are no surprises...

I wouldn't wait. You have a lot of equity, saving more is not worthwhile. The 17k as a reserve is perfectly fine. If there are no surprises with the plot, I consider construction costs and additional building costs realistic. You will get good conditions from the bank, maybe better than if you wait. There are contracts available where the repayment rate can be changed without problems.

Without children you have more peace and time for planning, selecting fittings, moving. I consider the rate of 1,800 realistic as long as you have no children. We have two children and pay 1,500. My experience is that the financial cuts caused by children are only minor if the woman goes back to work after a maximum of 12 months of parental leave.

At a rate of 1,800 EUR per month and interest rates of 1.06% or 1.46% (10 or 15 years fixed), after 4 years, that is when children are planned, there are still 267,000 EUR or 272,000 EUR remaining = still 60% of the value of the house = 40% equity ratio after 4 years. (already 70,000 EUR repaid)

If the salary does not increase in real terms but only keeps up with inflation, then he would already be earning 2,900 or 3,000 EUR net.

Parental allowance of 1,350 to 1,400 EUR from the wife’s also inflation-adjusted salary. Additionally, child benefit, and together the net amount is also 4,500 EUR.

Whether 10k more or less is assumed here does not change the basic statement. The important thing is to plan well and keep an eye on all costs.

My experience is that the financial cuts caused by children are only minor if the woman goes back to work after at most 12 months of parental leave.

When they are a bit older, yes, the financial cuts are limited. But that's actually not the main issue. Where expectant parents really freak out is with the basic equipment. Yes, you can get all that cheaply second-hand, but you usually want some things brand new, and usually that's terribly expensive. Children's furniture (there are really great ones) can easily run up to 1000€. A stroller from a higher-end brand (and yes, you can really notice the difference!) quickly costs around 500 € and more. If the woman can't breastfeed and the child, annoyingly, only tolerates Aptamil powdered milk (somehow that works best for the kids), you shell out 14 € every 3 days. In other words, the first year can be really expensive and then it can get very tight with the parental allowance, unless you have saved up beforehand. Afterwards, the woman might work again, but then there are daycare fees, which in some places are really outrageously high – especially for under 2-year-olds. And even now (our son is almost 2), unexpected costs come up. The kid climbs headfirst out of his bed... over the rail! It's not like he could climb out normally from his crib. So I’m just waiting to find him at night with a broken neck. So a new bed is needed. Since the kid is still too small for a normal one, we needed a special "transition bed." Luckily, there is one that can later be converted into all sorts of versions. Cost: 834€. Just like that, bam! That puts a nasty hole in the wallet and the savings.

Conclusion: Never underestimate the cost of a child! As sad as that is.