Radiant heat in ceiling

I owned a Condo a few years ago with radiant ceiling heat, similar construction timeframe (I guess warm air fell in the '70s). We did not have any ceiling cracks, but skim coating or texturing the ceiling if you avoid screwing anything in should work fine, based on the experiences of others in our complex.

Our unit had had the heat fail in a few of the rooms, and we simply had wall heater units installed instead of trying to replace or repair the radiant heat. I don't think that the ceiling heat was worth the trouble, because at the floor, it was still always cold in winter, given that we had a ground-floor unit and weren't getting the benefit of someone else's ceiling heat.

After checking what type of instant water heater that is - since it's gas-fired, your operating cost will be MUCH lower with gas providing the heat than with electric resistance heat. Perhaps 30% (not 30% less, unless you have remarkably low electric rates and high gas rates) - typically near 30% of the cost of running electric resistance heat, for the same heat output.

Electric can be a shade thinner, but it's on the order of 1/4" .vs. 1/2". In either case, you should build an insulated floor on the slab unless the carport slab was insulated (very unlikley) or you'll be trying to heat the outdoors via an uninsulated slab, so the effective difference in height is none, really. Depending what you want for a floor, lay 2" XPS (extruded polystyrene insulation) and embed your tubes in gypsum cement or something like that on top of it, or buy pre-grooved subflooring to drop tubes into (and then pay attention where not to drive nails, or just don't choose a nailed down flooring.) There are a number of common systems for adding radiant heat on top of an existing floor.