RE: How to get a perfectly flat (=plan, leveled) workbench?
What I've always used is a top made out of 1" marine ply. It's straight, and very stable.
I choose the best lookin' surface for the top (which is difficult as it all looks perfect) and the screw a 1"x2" frame all around.
I then lay that onto of a normal Chip Board constructed work bench and pack-up the corners as required.
What nice is that it's stone ridged and completely stable yet... and this is important... you can knock a pin into it... without bending 5 in the process!
Remember, Marine ply not just general building plywood.
It has a light Ash/Maple tone... very nice to look at.