I've got a VPI Scoutmaster on the top shelf of a Target VW-2 (along with a 60 lb slab of granite replacing the MDF shelf), and a phonostage on the bottom shelf. When I first installed it (into the studs in the wall), I kept a carpenter's level on the shelf, just to make sure I would notice any subtle sagging if it occurred. It's been over a year now, and it hasn't moved. Wall mounting is much better than joisted floors. I used to have to tiptoe around the rack when it was on the floor. Now I can jump up and down and the turntable is rock steady.
Nsgarch has good advice re using an exterior wall (or a load-bearing interior wall).
Nsgarch has good advice re using an exterior wall (or a load-bearing interior wall).