Another thought for a strong flat work bench.
All the designs here use 2x4s and 2x6s. 2x6's vertical joists are probably overkill for a work surface. 2x6 16" on center could probably support 3000+ pounds.
2x4s are plenty. You could, it carefully constructed use 1x4 material.

In a floor joist with solid blocking between the joists, current testing I've red about in FineHomebuilding mag rates it almost as high as if uncut joists were running in both 90 degree directions.

I think I'd rip some 2x3s or 4's on my table saw (to make 1x3s or 1x4s) lay down a sheet of medium density fiberboard the size I want for the top. Make sure it's flat and level.
Cut all my 'joists' and blocking, then mix up all the old epoxy I've got laying around, add a little filler (or pancake mix) to thicken it, then glue it all together.
A coat of sealer and it will be flat and strong for a hundred years.
for legs I'd just make some really solid saw horses. The only way I'd add regular legs is if it was to be in one permanent spot, and I wanted storage underneath.