I've had a 500c/m for about 15 years, and haven't used it nearly enough. I've recently started using it much more often, and am happy to say that the love is back. What a great system!
I recently bought a copy of the Hasselblad Manual, 3rd Edition, which has all the information relevant to a 500c/m system at the right price point. Ideally, you should try to get a copy of the Revised Reprint 1989 that has an extra chapter covering new camera bodies, lenses, and accessories introduced between 1986 and 1989. TIP: Whatever edition of the book you purchase, make sure it has all of the color pages intact. The first copy I bought had all of the fashion/glamour photos surgically removed via an X-ACTO (or similar) knife, so I had to return it. Even if you don't typically do this type of photography, you paid for a complete book.
As for lenses and accessories:
I really recommend the 42170 Acute-Matte focusing screen with grid and split-image microprism. I shoot a lot of urban-scapes, and find the grid invaluable for keeping everything aligned when hand holding the camera. The only downside is this particular focusing screen currently sells for a lot more than I paid 10 years ago.
I think having two A12 backs is sufficient. I used to have three, but found that I really didn't need to carry three backs around. Loading a roll of film isn't a major time issue unless you're shooting a wedding.
As for wide-angle lenses, I can't say enough good things about the 50mm Distagon f/4 CF FLE. I use this lens much more often than my 80mm Planar CF. I absolutely love it. There's absolutely nothing wrong with the 80mm; the 50mm just suits my style better. Also, I now only use CF lenses. For me, the CF useability is preferable to the earlier C lenses. I don't find that the later CFi or CFE lenses offer any features that I need. I guess I'm a CF die-hard.