Re: Tankless water Heater ?
Go ahead and build on your slab but spend a few more bucks and put a floor on to of it. It won't take much and it'll be a lot simpler and cheaperthan radiant heat in the slab. You also would have to use glycol in the system unless you plan to heat the place year round. Personally, I would put down 2x4's right on the slab for floor joists, Use a vapor barrier, insulate, and then use 3/4 OSB for the floor. If you want to have almost the same effect, lay firring strips down, use foamboard to insulate and then your flooring.
I did 2x6's (mostly to allow for enough fall in the drains) when I converted half of my one car garage into a nice size bathroom and the floor is nice and comfortable.
Life Member of NRA, Tall Tine outfitters, and life member of UBP.