Your local plumbing/heating suppliers will be able to sell radiant tubing and all the supplies. Radiant tubing manufacturers will also provide complete design plans at no cost for your addition or an entire home. You will then have a professionally designed system that you can install with confidence.
If the water heater is used only for the floor heat and not also for domestic water to taps, it has been used successfully - but treat it as a boiler and add 30 psig boiler safety controls with a high limit electric control, not 125 psi relief valves. Electric rates are high in most places, so electric costs to run a 24x24 (small house) would be high also. Insulate the bottom of the slab before the pour so 15% of the heat bill is not lost to the ground.
Consider investing at this time in a high-efficiency fuel-burning boiler to heat the residence and the garage as a zone, thereby reducing overall costs of operation that can pay for the new installation from the saving in heating costs.