I had a SW612 and the 55mm lens. Great camera. I found an optical accessory rangefinder for focusing close was very useful--you can pick them up on eBay.
There is fall off on the edge of a 6x12 format. I used a Schneider center filter. One of the nicest center filters out there. Stay away from the Heliopan Center Filter as they can have air bubbles in the glass and the distributor is horrible to work with. You will also need to shoot at f/11--the larger apertures will just introduce more vignetting. But the camera produces great images. 6x12 is my favorite pano format as it is much more flexible with different subjects and the SW612 is easy to handhold and carry.
The 55mm Grandagon is really the film lens that has been rebadged. It is a nice lens, but it is not that special. Flare when shooting into the sun can be a problem as well.
In order to us a P45 back with the lens, you will need the SW-D body. The lens should be OK with the back--I shoot the same lens with a P25+ back and the images are really very nice. Things start falling apart with large movements.