This should get you started :
http://www.photoethnography.com/ClassicCameras/index-frameset.html?Hasselblad500.html~mainFrame
Check out the lens differences too.
I would also say the 503CW is the one to get, then you can add in the CW Winder for enhanced handheld shooting, and the PME 45 prism finder (the PME90 will not allow you to use a Polaroid back later). Then later on the 150mm CF or 180mm, superb with portraits.
Try to get superb condition ones to avoid problems, even if the cheaper ones are tempting but with doubts on the condition.