the number that people are quoting is the weight of the diff fluid used in each of the three differentials going from front to back. For instance, I run 7-5-5 so I have 7000wt in the front, and then 5000wt in the center and rear.

I am running the stock motor and it works fine, no issues there and no hurry to change.

Wheels and tires you have a couple of different options. There are stock, then some are running stock wheels with the Switch tires like they run on the Slash and Slayer (I'm trying to get more info on this) and apparently you get better traction with the scale look then you do with the stock tires. Then there are 1/8 scale buggy tires. Which give you better traction and performance but takes away from the look and feel of the truck IMO.