For lenses, I would look for a CF lens, simply because it is newer, and probably has less wear. That said, if you find a pristine C with obvious low miles, it may be a better choice. If you do get a C lens, have it CLAd now, while parts are available. As to the 50mm lens, I would get the model with the floating element (I think its FLE). More expensive, but worth it.
For the body, again, I would look for a newer model, such as the 501C/M. Newer bodies generally have less wear, and they generally have the later model Accute Matte focus screen. But regardless of the body, I would have a good CLA performed. I dont think the TTL flash option is worth the additional price. If you want TTL, you need the 503, of for the motor drive version, the 553. Finally, you may wish to consider the motor cameras. The 553 uses AA cells, and the older 500EL uses nicads, but a converter that uses 9v cells is available. I would guess the motor cameras have had more use
For the back, get an A12 for 120. The A24 for 220 are a little cheaper, and can take 120 film, but the frame spacing is a little off, and you may only get 11 exposures. I like the newer backs, from about 1997 or so, that have the dark slide holder. The older backs work just as good, but buy the add-on slide holder.
I use a PM5 45-degree finder. Incredibly bright as compared to 35mm, and well worth the price. I find the prism much easier to use than the waist finder, but I have a history with 35mm. Almost mint condition PM5 finders are going for under $250 on ebay. Personally, I would not get the meter finderits too expensive and probably wont last as long.
Good luck--you will never regret the Hasselblad.