All kits can be upgraded. The main thing you want to replace when you buy a used kit are the bearings. Its hard to tell they are bad until they go out and then you have a slow limp back to the truck. Don't fear the used kits, just double check all the welds with a shop light and replace the bearings with greaseable ones TS sells. Its a great way to save some coin to get into the sport.
No one year over year upgrade places one kit above another. A 2011 ST kit will hang with a 2014 ST. I would say the biggest upgrade TS made was the track and ski change in 2013. The traction difference is noticeable with the track, but the ski upgrade was major. You absolutely want the new 10 inch ski... no doubt. Again, it is a part you can upgrade. Most of the other changes have been focused around comfort and trail manners.
Good luck!