RE: How to get a perfectly flat (=plan, leveled) workbench?
What most of us use is a hollow-core door placed on top of some type of base structure.
If you check your local Home Improvement store, you'll find that they often have doors with a defect (Maybe it has a hole in one side) that they will sell cheap.
This can be placed on top of an existing table, or across a set of saw horses. Or you can build a frame like this one