You didn’t mention a wheelchair, so I’d recommend an adjustable-height stool for sitting at your workbench. Get one with pneumatic lift for quick and effortless height adjustment of the seat. For some tasks you want to have a higher approach to your work than for others, so this gives you options. If you’re on portable O2 when you’re in the workshop, perhaps you could make a wheeled trolley/stool combo that holds the concentrator/O2 tank, and then you can move about your workspace without getting up.
For your workbench, I’d think about leverage. If you’re going to be doing most of your work from a seated position, and you want to be able to use your body weight/strength, build the workbench lower to give you the best access to your projects. If you’re mostly doing fine detail/finishing work, then build the bench higher so you can see your work better. But, remember… with an adjustable stool, you can have all the options.
What about dining table height? That’s typically around 30 inches from the floor to the table surface. Working backwards, tha’s 26 to 27 inches height under the top/inside the knee well for drawer space, which should be enough for one or two deep drawers on a side, and a shallower drawer for smaller things on either side.