I do not know about that specific Calumet model, but unless it says otherwise, it should include a back. I understand thta KEH have a good return policy so that should reduce some of the related risk, but a phone call or e-mail could also go a long way. Also their reputation is that they tend to understate the condition, as that would result in fewer returns.
Do understand that a Calumet 4x5 could be any manner of creature as Calumet are distributors and have sold their own cameras, Cambo's, and others just to name a few.
As well as needing the back on the camera, you will also need a lensboard, unless the lens on your Speed Graphic will fit on its existing lensboard and work for you. You have not indicated whether your Speed Graphic lens is mounted in a shutter or in a barrel, as that could impact how it all works for you.
And long bellows are not just for long lenses, as you can also do close up work. You might want to maximize the difference between the monorail and Speed Graphic (e.g. long bellows, wide angle capability, greater movements, etc. versus small compact package with built-in shutter), otherwise you might ask yourself, why bother...
Good luck with your exploration of that part of the world (intentionally ambiguous...)