Speaking of welding tables... I looked at those fancy Certi Flat tables but the price scared me away. For my first welding table I salvaged a small tool chest, added casters, and tacked a 2' by 4' 1/2in steel plate on top. My second table was a badly rusted 4' by 8' steel table w/ a 1/4in steel top that I found at an auction. I bought a new 4' by 8' 1/4in sheet and laid it on top. Not perfectly flat but good enough for me.
Being such a big table and my lack of space, it needs to be multi-functional. I try to keep it from getting cluttered so I can quickly convert it from a welding table to other uses. I throw a 4x8 plywood sheet and use as an outfeed table for my tablesaw and also use it for a layout table. Recently I bought a 4x8 sheet of 1in rigid foam insulation and use it as sacrificial backing to cut plywood with my track saw.
I also looked at those high priced welding tables when I purchased my Miller 140 AutoSet Mig Welder, but for me I would not be using it enough for the cost.
My first welding table (many moons ago) was a used banking cabinet on wheels with 2 drawers and 2 doors, that I added a piece of 1/2" plate to the top that overhung 2" on all sides.
I also have built a few welding tables, but now I find it a lot easier to just add some wheels and a top plate to a boltless shelf unit:
Years ago I purchased a half dozen of these at Costco and have put 3" HF locking caster wheel on all of them, they are 36"L x 18"W x 36"H.
I have mounted a drill press, table belt sander, vise, cutoff saw, radial arm saw on some of them, and added 1/4" plate on top of 2 of them for welding.
My table saw is also on locking wheels, mounted it on top of a wooden platform.
My vertical band saw is also mounted on a piece of 3/16" plate with locking wheels.
I like your idea of the 1" rigid foam insulation for a backing, might do that as I am currently removing older interior doors and installing new interior slab doors for the honey do list.
KC