I wouldn't bother with the premade panel for what you want, it is cheaper to buy the parts separately.  

You could still use a little water heater, I would just probably have a separate one for the floor and domestic hot water.  The one I'm using in my house was a $2000 stainless gas with the same modulating burner HTP puts in boilers, but I have a house full of girls.  

Here is a little electric on demand water heater for a couple hundred $$.  Probably not much difference in this and a true electric boiler.  I'd consider a pair of these.  One for heat and one for hot water.  

https://www.menards.com/main/plumbing/water-heaters/electric-water-heaters/richmond-reg-essential-reg-tankless-electric-water-heater/rmtex-06/p-1499409330734-c-8690.htm

I don't have antifreeze in any of my floors.  I figure if the pump is still circulating the water it will take a long time to get that cold enough to freeze.  Add a wifi thermostat and you can at least check in on it.  If you want a little more piece of mind, add an electric baseboard on seperate thermostat set to 40 degree's.