I used laminations of, from the top going down, 1 x 12mm construction grade ply, 2 x laminations of 18mm chipboard, 1 x lamination of 12mm construction grade ply, making a total of approx 60mm, all glued and screwed together. There are three benchtops like this, one with my "big" lathe a Sieg C3 (so not really big!!), one with a Sieg X2 mill, and one with a small lathe, a Schaublin T70. All the benchtops are approx 1,200mm x 600mm and are painted with two or three coats of exterior grade, full gloss enamel paint. I used white for several reasons, the first I had it, secondly most of my work is Al. Alloy which is inherently clean, thirdly for the lighting properties. If I was using a lot of cast iron for instance I don't think the white paint would have been such a good idea! The actual benches are those knock together steel benches available at most big hardware shops. I knocked them together and drilled lots of holes which were used to bolt everything together. That has been fine so far. The actual benchtops are clamped with long pieces of angle clamping the tops to the frame, from under the benchtop. Sounds a bit Harry Tate, but it's lasted six or seven years with no problems so far.
If I had access to a welder I would have used 50 x 50 x 3 RHS and welded up a substantial frame, then used a steel benchtop. This would all be painted white.
best of luck!!
cheers
Bill