-3Racing (and a few others) make an oil filled diff for the 05/06. It won't fit the 03/04.
-The Xevo is hard to find these days. You'll only find it used. It handles very well, but I've read that the ABC Gambado/Grid is even better, and it's still available.
-What pinion you use will depend on track conditions, motor, etc. the Ride/3Racing speed gears are crap, btw.
Where are you racing? Seems like the rules are fairly liberal.