are you talking about electric radiant like you see in a bathroom remodel or hydronic radiant (using water lines), which is what i have in my house. electric is a lot more expensive to run, but hydronic requires more equipment to install.
unless you know that there is sufficient insulation under your slab already, you def want insulation between your heated floor and the slab, or most of your heat will go into the ground. I would do an inch of radiant, plywood, electric heat, and tile if that's the way you want to go. on a large scale like that, you'll want to consult an electrician for load requirements, etc.
if you want hydronic heat and the initial expense of the pumps and such required for that, you would do an inch of xps, then they make plywood with knobs on it made to click in the pex water lines, and then you can put down whatever floor you want (that is approved for radiant heat - some laminates/engineered woods are, some aren't) over that knobbed plywood. this will be about 5/8" thicker overall than the electric option.
either way, if you're heating the floor, you definitely want insulation. if the house was built to have a radiant slab, they should have put at least 2" foam under the slab to keep the slab's heat inside, rather than heatsinking into the dirt.