Sounds like a decent start. Personally I prefer using the waist level finder and the hand crank but the best part of the 645 is that everybody can configure it the way they want it. I have a non-metered prism finder in the bag for times when I want to shoot vertical, but that's not often for me.
One thing, you cannot use 1 dark slide for swapping backs; each back needs it's own. The backs should each come with a slide as part of the deal, so I wouldn't sweat this. Only concern would be if the camera body comes with a back that is missing the dark slide. There is a mechanical interlock so you cannot remove a back unless the dark slide is in place.