Realistic financial planning for KFW40 prefabricated house?

The thing is almost a bunker and with that you really can't do anything other than tear it down... There are no connections inside, no water, no sewage, no electricity.

Aha, and how did all that get into the house so far? If the basement is really just a legacy of the property, you need an even better planner. A demolition site is an even more difficult basis than a ground-level building gap of pure neutral nothingness. You would probably be out of luck, betrayed and sold out, hoping for real help from a general contractor lackey. I don’t see any way around an architect. Working your way up from such starting conditions also costs money, not least. I had already pointed out the luxury KfW40. I can well imagine that large format tiles and the like would also end up on the candidates for elimination list.

Aha, and how did all that get into the house so far?

Not at all.
That’s what I meant by "No running water".

If the basement is really just a legacy of the property, you all the more need a good planner. A demolition site is an even more difficult basis than a ground-level gap from pure neutral nothingness. You’d probably be screwed, betrayed, and sold out hoping for real help from a general contractor’s lackey. I see no way around an architect then. Working your way up from such starting conditions also costs money, not least. I already pointed out the luxury of KfW40. I can well imagine that large-format tiles and the like would also make the cut list.

Yes, it won’t be easy, that’s for sure...

But maybe back to my question: What can be done regarding the basement with €70,000?

costs Timber-frame prefabricated house with 130sqm: 300,000 (?)

Do you really have an offer in this price range? I am currently also receiving offers in the Munich area and the prices are much higher.
I would be happy if you could recommend a prefabricated house manufacturer with a good offer.