There are a lot of different threads about upgrades....You can be reading for months.
I'm new to the SXS world, but after all my reading I started with these
Dalton Cutch kit - Helps with RPM drop and not the coin of all new clutches (Hunterworks)
Holtz Steering rack gusset
Alex at CT Worxs Front gusset kit and Tie Rods
Install gauge to measure belt temp
Sport mode low switch (enables full power in low range)
Harnesses are great! If that is in the budget
Check the vendors section like others recommend. They will take care of yah
It all depends on how much you want to spend. The upgrades I mentioned will be recommended by most on here