Make sure the heat pump is a ground-source one if you get real winters. Otherwise it’s only good for supplementary heating in the dead of winter unless it’s super overpowered for the size of the house. Mine is air since I got it mostly for cooling in the summer, it does the job in spring and autumn so I don’t have to burn any wood, but throwing 300€ a month at it only accounts for about a fifth of the heating effort in the cold part of the winter, need another ~400-500€ worth of firewood or briquettes to make up for the rest of it. Air to Water has a similar weakness as Air to Air and while those usually have resistive heating as a backup, that’ll quickly get you into 1000+ euro electric bill territory in the winter.