I have always built this kind of shop furniture out of steel, first making a welded frame out to square hollow section, and then cladding it.
In that situation I could always place the casters well inside the length (but rarely had to, for reasons of strength, of course).
Probably I would still build a frame with some structural integrity if I was to make a shop bench / table like this. I am new to this wood caper, but all the stress and strain flows through your joints, and I think with very little extra material it could be a much more robust structure. (A steel frame for your design would cost less than an equally strong hardwood frame, quick to build, and the rest could be wood. You wouldn't know about the steel until you turned it over.)
Anyway, I think what you're building will do a real job. Well done. It will finish up looking great.
Buy the best castors you can, two with locks.