I want to add, in addition to all the ideas for the science of the bench. Get some info and prices of cirtain tools for wood. Make sure your joints are not surface to surface but datoed or something. That will give you more surface area for sealing. Not only that but the joint will be stronger. Then you will know you will not be chasing leaks around when calibrating your bench. There are lots of ideas of bench builds in the projects area. There is also Bruce's plans which would give you a good bench and take some of the fun and frustration of designing it yourself.
Mark, as for the plates. Are you using manometers or a computer. And what kinds of heads will you be working on.
Chris