Condition, condition, condition! From the K3 forward, the basic mechanicals were settled up and there were mainly stylistic differences. The MX-EVS has the larger focus knob, and I believe the aperture is 5-bladed, not 10 as on earlier Synchro-compurs (not exactly certain when the Synchros went to 5 blades). The base plate to accept a Rolleifix. Double-walled back to avoid foam or yarn for light seals.
One thing on the '3' to note is if the lens has a red T (Zeiss coating) or a red triangle (Schneider) indicating coated lenses. At some time in the '50s this stopped being indicated- I guess the assumption was that all were coated? I'm not sure when the coating symbol was dropped.
Condition. And cosmetics don't tell the full story. I have a 2.8C Xenotar that looks pretty bad but has been overhauled and aligned. I've had very nice looking ones with crunchy focus systems, etc. All in all, I suggest buying from a place like here or rangefinder forum as people will be more prone to be honest and to stand behind their reputation. Ebay is a crap shoot.