I must continue to resist buying one of these to relive the joys of analogue (there's a Nikkormat 35mm upstairs for emergencies), but if I ever do succumb, it is as well to know what to get. Ken Rockwell (as usual) has a stong opinion on this and I tend to believe him.
He recommends the 501C or 503CX because the screens are brighter than the "otherwise wonderful 500 C/M".
Get a prism, ideally the PME-style Metered Prism.
On lenses, he recommends the CF series and avoid the CBs.
And finally, don't forget, DO NOT BUY ANY OF THE ABOVE.