lasteagle, I'm glad to hear someone else suggesting warehouse racks. I bought four uprights and used two of them with crossbars to frame the benchtop-area. I got bored and wanted another area for mounting my stationary tools, and as the pallet rack uprights were set up such that the bench would be 36" deep, I cut up the uprights and re-welded them to make a 22" deep section and welded my tools between the two crossbars attached to it.
Although wood is a great idea for a bench top, don't limit yourself to it until you've thoroughly decided you don't want a metal worktop. Granted, there are thousands of benefits to a wooden worktop in comparison, but I happened upon a deal on Amazon selling a Global Industrial product at a ridiculous discount. It was a section of square-edged, reinforced 12-ga steel with C-beams stretching between the long sides. Specifically manufactured to be a workbench top. Although I admit it took the fun out of fabricating the benchtop myself, for once the pre-designed product worked out VERY well. Besides, with all reinforcement C-beams installed, the steel sections are each rated to hold 3600 lb dead center...