I'm a bit stuck in the past with Hasselblads as I have a 500C. I don't think that there is all that much difference between the 500C and 500C/M (which means modified) and all I can see is that the focussing screens are more easily interchanged on the C/M. My C was the last yer of production before the C/M and has the newer type sliders for screen changing, so go figure!
Some others here will no doubt fill you in on the later models like the 501C, but I think that there is precious little difference in these models...change comes slowly at Hasselblad...but if you are on a winner just stay with it.
As you say, the 80mm Planar is usually included, which is a great way to start...top lens, and after the 80 people usually add the 150mm Sonnar (one of the all time great portrait lenses) and the 50mm Distagon. These two lenses are very common and should be reasonably priced. In the early seventies the Zeiss lenses were multicoated, as indicated by a red* and are referred to as T star lenses.
Most of my hasselblad lenses are just the single coated versions and I find no reason to upgrade the the T stars. The wide angles do benefit somewhat from multicoating due to the complex multi-element designs and the fact to you are more likely to include a light source.
Check the camera carefully before laying down your cash as a lot of Hassy's have been used professionally, and the leaf shutters on the older lenses can run slow. especially on the slow speeds. I don't want to labor that point though as the Hassy's are really built to work.