Heat pump: Which makes sense? (Viessmann Vitocal 250-A?)

It's a fresh water station. I’m not very familiar with whether they also come in smaller sizes.

But the offer is way too expensive for me. About 30-50% less should be possible.

Are you referring to the setting or the individual prices (more expensive provider, you can get the 250 cheaper, etc.)? If setting: Where could one save?

I understood that the buffer tank is important for the efficiency and lifespan of the heat pump. What did you mean by "bigger challenge to tackle, if it says so at Viessmann"?

I understood that buffer tanks are important for the efficiency and lifespan of the heat pump. What did you mean by "drilling a bigger hole if it says so at Viessmann"?

The exact opposite is true. A separation or parallel buffer causes more short cycling = less efficiency, more wear.
A small serial/in-line buffer in the return flow can be useful, for example, with radiators and air/water heat pumps for the defrosting process.
And hands off ERR. The heat pump doesn't like this throttling either.

Are you referring to the setup or the individual prices (expensive provider, you get the 250 cheaper, etc.)? If setup: Where could you save?

I understood that the buffer tank is important for efficiency and lifespan of the heat pump. What did you mean by "bigger fish to fry, if it says so at "?

I was referring to the buffer tank. Expensive and causes inefficiency (although that should be seen in relative terms).
The best heat pump is the one that simply goes from the heat pump into the underfloor heating and back. Properly calculated and designed and running as much as possible 24/7. Not slowed down by individual room controllers (you can be exempted from them).