Looking for Answers about 2x4 Basics Workbench Kit, Model# 90158?

No. I built my workbench 2'X8' with 3/4" builder grade plywood for the top and bottom shelf. I wanted the room on the bottom shelf to store larger items like my pancake compressor etc that would have made the second shelf in the way, so I left it out. It made no difference, this bench is so sturdy I can build a tank on it without it sagging. I will say, my bench is narrow and long so I cannot attest to the need for the second shelf if the bench is built different, but with the cross braces in I would still build it without the second shelf.

Yes. The bench can be built with horizontal brace under each shelf. I built mine 2' X 8' with 3/4" contractor grade plywood for the top and bottom shelf, no middle shelf, and did not use a horizontal brace under either shelf. The bench is so sturdy I can build a tank on it.

Yes there are shelf brackets in corners to rest the diagonal support for all three shelves. However diagonal bracing is not my choice. I used perpendicular bracing every two feet instead. Just seems a sturdier way to frame.

leg design has built in ledges for the shelfs to nest in. The top lip is 31 inches from the base of the leg. Add the 2x4 top layer and the top shelf rests on 34.5 inches from ground. You can vary the height of the workbench surface if inclined .So ADD .50 inch plywood an at 35 inches etc

You can, and I was thinking of doing so, but instead I move mine with a hand cart. Even though you can lock casters they never stay as stable as without.

Yes, I assembled mine with only the bottom shelf which is approximately 6" from the floor. The middle shelf doesn't mount with as many factory drilled holes through the legs so stability may be an issue.

The lower shelf will store an item up-to 11" and the middle shelf up-to 9" in height.

This workbench kit includes brackets to make square or rectangular workbenches. The brackets would not work for the inside corner of an L shaped workbench.