You want to look for the ProS model which had the film/shutter interlocks. Also ask about the felt seals on the film backs. In lenses, you want to look for the "C" type, which were better coated/more contrast. In the RB's a 90mm/127mm is a standard lens, 65mm is wide angle. The 150 soft focus (make sure you get the disks) and the 180 make very nice portrait lenses. The RB's (and a lot of the 6x6 and 6x7's) used a leaf shutter in the lens. The top shutter speed was 1/400 and was rarely accurate, although the slower speeds were usually close enough. The RZ's used an electronic shutter, but I think they're still spendy since you can fit them with a Leaf back.

The RB was really a studio camera. I dragged mine into the field as it also made an awesome landscape camera but I got a good workout doing so. The 645's are way easier to handle, but I wanted a bigger step up from 35mm. There is NOTHING like looking down into that ground glass with both eyes. The 6x7 negs are pretty amazing too.