Being on the latest release is something we would always advise, and yes will always make for the smoothest upgrade process
Yes, we always aim to make the process a smooth process. But this is also a large upgrade. If you have custom items, you need to account for those not working and needing upgrading when you get to that point
Review all 3rd party items and remove what you are not using. You could indeed set up a test environment as mentioned above, ready for this. However I would advise probably doing this closer to the time. Other than that, there isnt really anything else you can do to prepare until we are ready.