Air-to-air heat pump, yet radiators?

Yes, I'll do it. The information comes from the seller...so it can be assumed that it is correct. At least for the preliminary research .

The costs of the heat pump should be around ~100 €.

Expensive fun

100€/month of course ...forgotten.

Are you prone to colds or do you have other health problems with air that is too dry? Then hands off. Otherwise, it probably depends on the price whether you accept this actually inferior heating or not. What is the energy standard? At kfw40 or better, it can be okay.

Since the house is supposed to cost well over ~ €500,000 all in - I am not willing to accept an inferior heating system.

Do you know a smart site to generally get an overview of heating technologies etc. including evaluations for a layperson?

Many thanks

Here in the forum there are some people who know their stuff and additionally you have Erfahrungsberichte from those who have already built. So you can get help here.