Nice catch! The lens you have should cover the roll film back without any problems, certainly without using movements. I don't know what kind of circle it had, but as I recall, the movements on the weren't too extreme.
Lens prices have gone down just like all other film equipment, but you still have to watch to get the bargains, just like you did to get that baby. Also be prepared to have any used lens CLAd, which at the least will be $65 to $75. You won't know until you test it. Worth it if you get a good enough deal. No matter, I think you are going to like this camera, and will probably use the roll film back more than you might think just to same film costs, and still have a fairly large negative.