I think I understand what you're considering. I built the display cabinets shown below against an existing wall in a similar fashion to what you are considering. The left side base and the right side cabinet were built in place, using the existing far right and far left side walls. The lumber I used was paint grade soft maple - as I got it for extremely cheap. The shelves have adjustable positions, using the standard brass pins for placement.
Edit - 48" spacing for shelves is quite a distance for plywood - even with a center rail for support.
