My bench top is 3/4 PT plywood, 2 layers thick.
Same base material for mine, fastened together w/carpenter's glue & screws. It's topped with 3/16" steel plate. I looked at 1/4", but decided it wasn't going to accomplish anything extra, except be heavier and more-expensive.
I through-bolted the steel to the plywood, using 1/4" flat-head machine screws. A quick wipe w/an oily rag keeps the rust away. I may use a piece of masonite on-top if I have some surface I'm working on that I don't want to get dirty.
I used some 1-1/2" x 1/8" steel strap for the edges, the strap is screwed to the 1'1/2" plywood under the 3/16" steel top. I had the 3/16" steel top cut slightly-wider than 24" so the 1/8" strap would neatly fit underneath the front 3/16" 'lip,' and at the back I screwed a piece of painted steel about 4" high to the plywood top, the painted steel piece came with the workbench frame.
Beneath the frame are three Craftsman roller cabinets, two are drawers, one is twin-doors. They fit there perfectly, and I can easily remove them for cleaning behind/underneath.