The IKEA Antonius system is on wheels, if you want. Depth is 21" - width is 17 1/2 " - height is 30" with wheels/27 1/2 " without wheels. They stack really well too.
You could have shelves in the back of the closet and on one side and the Antonius in front of the shorter shelves. Since the Antonius does roll, you can move it out to get back at the back.
You might even want a tall area without shelving if you want to store your ironing board or a big board that fits on your ironing board or a hanging area.
Do you want some sort of a counter in there?
Otherwise, I'd vote for shelves that pull out so you can see and get at what is in back.
More questions than answers at this point.
ali