Interesting project! Do you have a link? I also plan to scan all my films using such DSLR
I think all can be done within the Arduino. There are plenty of examples to drive motors and LCD, read keys, communicate with PC through serial or USB... Unless you really need a complex GUI, I would not use anything else. You will then have a compact, autonomous and robust system, easy to debug. It will be more an integration exercise than a full dev.. That's the nice point with Arduino
And the Arduino community is so large that you will get all the help you need.
Canon 20D + 17-40/f4 L USM + 70-200/f4 L USM + 50/f1.4 USM Merlin/Orion panohead + Papywizard on Nokia N800 and HP TC-1100