Because your floor is relatively small and is not going to be your primary heat source, I'd probably not get all super wiggy about the performance overall, especially if you heat the water with solar. If I were doing this project I'd put my money and effort into harvesting and storing as much solar heated water as is practical and then sip that energy as needed. This would require more collectors/storage tanks but then you can use any extra energy for domestic hot water or even add a heat exchanger to supplement your forced air system.
1" foam on the slab and stuck to the walls*
(spray foam your sill boxes too while you're at it) would make the space at least somewhat decoupled from the "outside" but if you approach it from this angle (more solar!!) then your losses to the less than insulated slab would be not as painful as if you were paying for fossil fuel on an ongoing basis.
*my advice to all homeowners and builders: no fiberglass insulation anywhere in a basement ever!
Here's a graphic I put in all my energy evaluation reports
Good luck!