What model are you looking at?
We've got a 2013 Sightseer 36v.
I'd suggest just driving it for a while to see if it needs any of those add ons you're reading about. If it drives and handles well, like ours does, just leave it alone.
With the possible exception of doing the Cheap Handling Fix seen here (LOTS to read here, maybe start in the middle and work to catch up). Cheap Handling Fix - iRV2 Forums
In general, 5 speed is 'better' than 4 speed, 6 speed is 'better' than 5 speed.
BUT, you can't get a 6 speed unless you move to newer models than 2013.
Again, if you like it, drive it.