Good comments on the heating system. I'd add one more comment there, although Billy probably only works on high quality stuff so didn't mention this, is to have electronic ignition rather than a pilot light. Same thing if a water heater is to be installed. Too many flammable vapors in a garage environment to risk ignition from a flame.

As for the welding question, I've been asked this before. I've used both my plasma cutter and welder "over" the floor, not directly on it. These have been in light duty service, so probably didn't "test" the floor's resistance to damage. For a coating that cost me less than two bits a foot I wouldn't expect it would resist a lot of heat/flame/slag/whatever. My simple, and cost effective, solution is a welding blanket. You should have one anyway, and a good one can be had for less than $100. Secondly, my point of view is as a hobbyist. The frequency of my welding and cutting is relatively minimal. Were I in production welding, or looking to coat any industrial surface, then yes, I'd go with a professionally applied surface. It all comes down to return on investment. For the home shop I'd have to ask, why?. And this from a guy who is jabbed from time to time about recommending the higher cost (meaning quality) option most of the time. And Lord, thanks for your consideration. No harm taken, I'm pretty thick skinned. Even Streets can't offend me!!