I am new to tractor ownership which means the only experience I have so far is shopping for deals and used machines. The CK3510 was a new model in 2015 so they should be similar. I found that used tractor ads did not represent the machines well. The 2016 may be an SE model which would have bonus features over a straight 3510. It might have rear remotes not mentioned. The 2016 unit should at least have the 4010 or 4020 loader but check. In Canada, Kioti started offering an extended warranty for machines sold after Sept 1. In any case a 2016 will have some warranty left.
Obviously, ask about service records, warranty issues, leaks, damage. Confirm that they have the owner's manual, and maybe a service manual - bonus. Ask them who is the sevicing dealer and then call the service department to see what they think.
All in all, a well maintained 2016 may be a great deal!