With storage glycol adds a lot of money to the system and it don't transfer heat as well. Your better off to spend the money on a battery or generator backup IMO. I don't Know the formula to figure the temps you need to run in your floor but maybe someone else will chime in. There are a few ways to handle your DHW. You can add a side arm to your electric water heater. You can use a flat plate and flow switch for an on demand type set up, you can have a coil in your boiler or storage tank, or you can get an indirect water heater. the boiler water will always be the same water and never touch your DHW.