Especially in connection with the planned stove + KFW40, this should be perfect for an air-water heat pump. If the installer designs it for 28°, it could become difficult to impossible for the brine-water heat pump to recover the additional costs.
If you can live with the outdoor unit of the air-water heat pump, then certainly. Additionally, I believe that the cooling with an air-water heat pump is almost purely electric; with a brine-water heat pump, 2-3 degrees are probably lost through PC.
That is the big question. We do not like the outdoor units nor the ventilation slots on the house wall that are required for an air-to-water heat pump. Therefore, the choice of a ground-source heat pump would essentially be based on aesthetic reasons.
Alternatively, we would like to place the outdoor unit behind the garage, although that is actually also a nice spot for a wood storage...