RE: How to get a perfectly flat (=plan, leveled) workbench?
STONE IS THE WAY TO GO!
Look around for a damaged granite or marble kitchen worktop. I got mine at a the UK equivelent of a thrift shop for $5.
I then used contact adhesive to attach some 1/4 thick cork sheeting. Two layers of 1/8th tiles work as well. This lasted for years. Just make sure you get high density cork usually called flooring grade.
Yes it is heavy but it is absolutly flat and stable.