B&H and Adorama sell the Mk. III, new, for $848. Keh.com is selling a Mk. IV for $792 Excellent Plus rating or $822 Like New Minus rating. Do I risk getting a used Mk. IV or do I just get a Mk. III new? The Mk. IV has some higher specs but none are my reason for seeking an upgrade from my Mk. I. Whether III or IV, I like the connectivity (wifi and NFC) as well as the f/1.8-2.8 lens. I also have a Sony smartphone, which I look forward to using as a remote release with monitor. I only have apprehensions about buying used. What are your thoughts?