4 people; I believe a 3rd child is not planned, only in Kaho's draft were there 3 rooms.
You can throw the draft for your plot in the trash anyway, it doesn't work on a hillside plot, so forget it.
Region aside: you have a hillside plot. There are really awesome possibilities, but they are usually not standard and therefore more expensive than average.
In addition, a hillside plot always generates earthworks, no matter in which direction. That also costs money.
120sqm for 4 people works, but does it make sense when building?
You are in your mid-40s, you still have some time to pay off. So think about what you can really invest.
The cheap standard draft that you find on the prefab house providers' websites ("This Betty house 08/15 from €138,000!") you will have to completely forget about.