Hello,
Solar for hot water and radiant flooring yes, it is a good upgrade. Insulating the entire envelope is also good. This means the slab also. Using foam insulation is also a good upgrade. As to solar electric; it will not pay for itself unless you have a super efficient house design. Sealing the rock to the studs will seal walls from air leakage. This is not normally done in Texas. From a recent study, using a thermal mass inside of the house with typical insulation in the walls, reduces HVAC bills. Using cf lighting that has insulated cans when in the attic or uninsulated areas also helps.

Like Bob says, the return on investment for a solar hot water system is short and therefore is a no brainer. The return on investment for solar electricity is a LONG time. Consider wind power for electricity

Definitely do the solar hot water. I recommend the evacuated tube systems.