Solar Source Heat Pump

We have lost two of our tenants, which severely decreased our income. That postponed all remodel work. In the mean time, I have been thinking of different heating systems. One is to installed a closed loop water source heat pump that would extract the heat from the solar tank. The solar tank would be installed below the solar collector, with a capacity of about 2,000 gallons (30FT*3FT*3FT inside dimensions). The cold side of the heat pump (evaporator) is a heat exchanger that uses water from the solar tank. A mixing valve delivers a water temperature within the range of a standard heat pump, usually between 32 and 90F. The hot side is another heat exchanger, connected to the hydronic system.
The hydronic system could be smaller thanks to the higher water temp delivered by the heat pump. Baseboard heating would be adequate for such a system. The lower cost of the hydronic heaters offsets some of the cost of the heat pump. Some of the heaters, downstairs for example, could be home made using copper tubes.
The advantage of adding a heat pump is that more heat can be extracted from the solar tank. The colder water temp in the solar tank also increases the efficiency of the solar collector. The system will be able to run on solar for a longer period of time, but will also draw more electricity.
Alternatively, the most cost efficient system maybe a water source air handler type heat pump, that would re-use the air duct system in the house. This completely eliminate the cost of the hydronic system, and still give very good efficiency.
With a common rule of 600sqft/ton, we need a 4.5 to 5 ton system.
Another improvment of the heating system is to add zoning with these [ motorized thermostat controlled registers ]. The upstairs bedrooms and downstairs living room usually get overheated, so these rooms would get the thermostatic registers.
Ultimately, cost constraints will dictate the system, so my feeling is that we will probably go for a standard air-based heat pump with thermostatic dampers in strategic locations. Later, a solar heat collection system may assist the air source heat pump.