Buy from KEH in Atlanta. Read their return/guarantee policy, it's top-notch as is their customer service.
Later model 500C/M comes with the rapid winding knob, a sometimes useful feature. You may be able to find one with a Acute-Matte screen, which is much brighter than the original screen. This is a good thing.
Go for the CF lenses, C lens repair parts are no longer made. Focal length choices are entirely up to you, everyone seems to have their own opinions as to which they like.
Plan to pay for a clean/lube/adjust for every piece you purchase. You then will be set for a decade of trouble-free shooting.