I don't see disability insurance as a necessity for building a house, but rather for starting a family. Personally, I wouldn't mind not having to leave the house without children. However, when planning with a family, it really makes 100% sense. As a side discussion: not only consider "possible interest rate increases," but also inflation. I just entered it for fun into an online calculator. At retirement, it comes out to a purchasing power of about €680, assuming a current rate of €1200. That's the advantage: while wages will hopefully at least adjust to inflation, debts are subject to it.
Okay, even then you won't manage with 300k. For the 130-140m² size, I think you should manage with 350k, WITHOUT incidental construction costs, of course.
So if we take a look at MyHammer to see what the shell construction costs are. The price per square meter of living space is calculated there to be between 500 - 750 euros. Including civil engineering, roof, ceilings, masonry, foundation slab, etc. For a house with 160 sqm of living space, costs would amount to 80,000 € in the cheapest case and 120,000 € in the most expensive case. Including materials. So, now someone explain to me where the other 300,000 € disappear to?