Only the Mamiya Super 23 offered back movements -- and generally the Super's sell for a premium. Much more cost effective is the Mamiya Universal Press (although truth be told, complete systems sometimes can be quite expensive too)
As a long time Mamiya Universal user (25 years) I really like a rangefinder, but yes it will be expensive to collect a complete system, the 250 mm lens is very rare, finding the viewfinders for the wide angle lens can be a challenge. I have seen a lot of backs on auction sites, along with the ground glass back and few lens, but over the past couple of years it seems to my subjective experiance that the number of classic Mamiya lens and backs is dwindling. At least in my market finding a repair tech is diffcult, last time I needed my 150 lens adjusted I had to send it to a camera repair company out of state.
If you want a MF camera camera or system that you intend to use a lot, I would give some thought to a newer model, if you are like me a weekend user then a classic model may work well.