Keep in mind that the RB67 was developed and "hard used" as the standard workhorse of studio photography. There is no question as to its place in portraiture right up to its displacement with digital systems.
Pre-C lenses are certainly a bargain--and fraught with possible issues. One of the common ones is separation of the cement in the lens elements around the periphery--and problems with much slower performance at the highest speed of 1/500. Rarely are they worth the cost of reconditioning due to the current exchange values. The coating issue also presents--and can make for real problems if the camera is taken out of the studio environment.
The KL series requires an adapter--and rarely will one actually note a difference in performance with the C series. This latter series is affordable, a huge performer, and less likely to be subject to problems as the non-C lenses. The coatings match well to standard studio lighting solutions and also work well with field usage.
Of course, a lot of RB bodies and lenses have been beat to death and hung up fairly useless. The best venue is through a retailer such as KEH who vastly underrates the condition of a lens, and provides protections to the buyer that are not available in such places as Fleabay.
IMHO, if you leave the comfort of the studio and attach a grip with release, a prism is a sheer nuisance. Field use off the tripod is best accomplished with a waist level finder and a good depth of field. Nothing really beats a quality handheld meter either inside or outside the studio....