Daikin makes air source heat pumps with water storage, and that can be paired with solarthermal: - https://www.daikin.com/products/ac/lineup/heat_pump - https://airconditionerlab.com/daikin-heat-pump/

A "variable speed compressor" is what you want in a heat pump, as these modulate their speed (and electricity usage) to produce a constant amount of heat. You might be okay with a 24K BTU system, especially when coupled with the thermosiphon and then some effort spent on insulation & air-sealing?

Here in the US the base system would be about $3K USD, but with installation costs on top of that.