Which heating system is suitable for the new construction of a KfW 70 house

....And here the proposed gas condensing boiler in combination with solar thermal ...

Here it is probably more the intention, the father of wishful thinking ;-) Gas condensing is not bad, the investment is on par with, for example, an air heat pump. Unfortunately, you cannot cool with it, but you can cook. To meet external framework conditions, one usually has to additionally purchase a solar thermal system that is often uneconomical in a single-family house, "in accordance with regulations"! Now the reader may decide based on the demand balance (capacity, energy) which solution he prefers ;-)
Best regards