Take a look at the 501C too. It's usually quite a bit cheaper and has everything that the 501CM has except the floating mirror (which gives you a full image on the viewfinder with lenses longer than 120mm, older models have some cut-off with those lenses, but note that's only in the viewing system, the film place isn't blocked).
Someone's advice on the C cameras not having user-changable screens isn't correct, the 501C has user changable screens. The "M" in CM just stands for "modified," or at least that's what I have been told. the 500C came out first, followed by the 500CM, 501C, and then 501CM.
My Hasselblad kit consists of my 501C, 80mm CF, and two film backs, and that's all I've ever needed (though not necessarily all I ever wanted!). I prefer the black trim that the 501C comes in (I believe it only comes in black trim) and I knew I would never need or want a long focal length lens (at least not for my Hassy) and the 501C came with an acute-matte screen, whereas older ones didn't, so those reasons, paired with being cheaper than the 501CM resulted in my purchase of the 501C. Also, this let me spend more money on the lens, which is the most important, and sadly, expensive, part of the Hasselblad system.