Considering a used Pan Am as a second bike to add to the stable next to my Road Glide. If I find a deal on a '21, are there any concerns over a 22 or 23? If from a dealer (which it would be), as long as all software updates are done, would it be equal to say a 22?

 

22 and later can support the speed shifter. I believe there was also a modification to the wiring harness in 22.
It's usually always better to get the latest model your budget supports.