Welcome to the Fun House ... The advice is worth what you pay for it. We are all good with tools and computers
Yuor design is interesting. A portable bench? The casters will have to lock or it will run away from you. You can "block" the casters with 2x4s. Your top does not have to be plywood. You can use cheaper materials such as OSB. But I would attach a sheet of Hardboard (HDF) with the smooth side up on top of your OSB. It can be flipped when it gets messed up (end for end) or replaced. Cheaper than replacing a complete plywood top.
The height should be coordinated with the placement of your electrical outlets. Why the step-down? Coordinated with a tool like a miter saw?
Keep working on it! Should be very useful in the end.