Ground floor made of concrete with a height of 4.5m

that would be pretty sloppy and has nothing to do with proper sealing. Your budget might barely be enough for a 150 m2 bungalow with many compromises but not for the basement, excavation, and other incidental costs. Especially in Bavaria, where it is as expensive as in BW. A rough estimate for the basement at that height is 40-50k until you are done. What about the excavation? Will it remain on the property or does it have to be disposed of? Do you already have a soil survey? It reads as if it is just an idea so far but nothing has been planned yet....

4.5m height is like 2 basement floors stacked on top of each other.

It is often much cheaper to only construct the upper of the two virtual basement floors as such (in traditional masonry construction) and to heap a 2m high mineral aggregate bed underneath.

Basement walls must withstand lateral pressure forces in the earth-contact area; the higher the wall, the more likely it is to buckle. That means either many interior walls as supports for the exterior walls or several ring beams in between or, or,... I believe this could overwhelm normal single-family house construction companies. This is industrial construction.