If it's in good shape, and has the grip, that is a good price if it is a Universal or a Super 23. Not so much a good price if it is a Press/Standard. I would not trade your Rollei for it in any case.
You don't need separate viewfinders for the different formats. 6x9 and 6x7 are both marked in the viewfinder.
I would also immediately pick up a ground glass back for the camera. This is incredibly helpful.
I have a Super 23 and a Universal, as well as the Mamiya Professional Systems Handbook by Robb Smith. If you get the camera, and want a photocopy of the Press system chapter, let me know, cover the copies and the shipping, and I will send it to you.
The biggest problem with this system is that lenses and accessories tend to get expensive, as they are fairly rare, and there is a group of people who are nuts over these cameras. Your first kit (body, normal lens, etc.) will likely be reasonably priced, but backs, viewfinders, and lenses will be more.
2F/2F