Old gas heater defective -> need a new one, but which one?!?

What kind of heat pump is supposed to be installed that it should cost €26,000? Or is it a ground-source heat pump including drilling, etc?

Air-to-water heat pump with manager and separate domestic hot water heat pump including materials, installation, etc.: €26,360 gross
The pure air-to-water heat pump is E-price €11,075
It is a Stiebel Eltron WPL 25A

would a combined heat and power plant be an alternative?

Would a combined heat and power plant be an alternative?

As I said, I would be open to alternatives.
Strangely, no one offers something like that. For whatever reason.

Does anyone have experience with it?

It's more or less special and not every heating company offers something like that.

would a combined heat and power plant be an alternative?

For a small single-family house? Nah, what's supposed to be attractive about that?