RE: How to get a perfectly flat (=plan, leveled) workbench?
I didn't really read what's above, but I'm sure it's to the effect that there are all different ways to get things level. Plywood is designed to resist warping, and if properly supported will make a fine surface. I personally use a hollow core door because there was one laying around and it fits in my small space. I have it over a pair of tv stands (which gives me storage under the bench and a place to tuck my legs/chair into) and have the door shimmed level. It doesn't stay permanently perfect, but the times I actually need a perfectly level surface are few and far between, and just require a little reposition of shims to fix.
Next plan is to cover the bench with glass.