Heat pump for KfW55 house 148 sqm

Dear heat pump experts and experienced home builders,

I would like to be able to estimate which heat pump capacity we really need.

Our project:
2 full floors
No basement
148 sqm
KFW55

A 6kW air-to-water heat pump is suggested in our energy demand calculation.
However, the heating load calculation shows an 8kW air-to-air heat pump in the plan.
Both calculations are rather standard, since it is a developer.
Which information in both documents should I pay attention to?

8kW seems a lot to me, but that is just a gut feeling I developed by reading here in the forum.

The underfloor heating is already installed, also standard, the screed has been resting for 4 weeks and is waiting to be heated up.
Alone, no heat pump there.
The developer would credit us €15,000 if we take care of the pump and installation ourselves.

All these questions overwhelm us.
Is that even possible? Can we use a different model, company?
Is it worth it, is €15,000 okay?
Are 6kW enough?.....
We need ammunition to be able to stand our ground against the developer.

Help!
Thank you!

So I would not procure and have the heat pump installed myself. Any potential problems with the heating in the future will be blamed by the BT on the heat pump and its installation. Additionally: What happens if the heat pump suddenly becomes unavailable for you? That would be too risky for me.

Mono or split unit, which one are you installing? What does the heating engineer usually install? Do you know anyone who would install the pump otherwise? 15k could be tight including installation, maybe you should try to do it the other way around: you provide the pump, he installs it and pays for the pump... or you agree on something in the middle then. Btw beware that if you provide the pump you are also responsible for its foundation... we had to learn that the hard way.

We want to explore everything first and look in all directions. Planned is a Daikin Altherma 3 R F split unit. Whether someone will install it for us still needs to be clarified. We are also aware of the issue with the warranty and it doesn’t sit well with us. The developer gets off quite cheaply with this option and shifts the responsibility onto us.

Do you mean the foundation for the device inside or for the outdoor unit?
Inside we already have screed, I don't know if anything special is still planned there. We wanted to do the foundation for the outdoor unit ourselves anyway.

Exactly outside

Mhm first find someone who installs the heating, otherwise you'll be standing there afterward and no one will do it