I have searched for hydronic coils to use solar heated water in our furnace. This coil appears to be the best for the cost:
At 230kBTU/h, it should provide some heat, even when the solar heated water is down to 110F. The static pressure on the blower may be a problem. I could also add a second coil, one upstream and one downstream of the blower, balancing the pressure while improving the heat exchange capacity.
Here is a page giving derating from lower water temperatures:
Water temperature derating
Assuming one coils as above (230,000BTU/h at 180F), the output at 140F will be 131,000BTU/h. At 110F, we can estimate the derating to be 0.28. The heat output will then be 64400BTU/h, or more that 5 tons of heating, which should be enough.
The coil will be installed below the blower, where there should be enough room. The assembly that will hold it, will also hold a paper filter upstream, to prevent accumulation of dust in the coil. The coil+filter will replace the old dirty filter that is there now.
I also found, from builditsolar.com, a source of solar tanks of big capacity:
The best fitting tank for the furnace room is the 420 Gallons tank, measuring 43" x 76" x 54". That room could handle a 48" x 96" x 48", but that size doesn't exist standard. 420 gallons should provide enough storage capacity for most of the year. The tanks can handle 200F.
For the hot water system, their 110 gallon tank, at 34" x 34" x 54" will fit nicely. It could be coupled with a tankless electric water heater. The tank plus heat exchanger cost less than $2000. I'll have to compare that to other manufacturers. There is also the possibility of building it myself, which should be doable for the hot water tank, but difficult for the space heating tank, due to its bigger size.