Expect to replace the steering servo, spur gears and differentials since they are plastic. They do send the upgrades now, but you still have a plastic spur which you can ruin in one run.
I HIGHLY suggested you buy the kit and build it. This way you understand every part of how it works and if a problem occurs you know what to do. The X SS also has many upgraded parts out of the box, this will keep you from having to buy them later.
The money you would use on upgrades you can now spend on a high quality radio system and servos.
This may help you a bit, my build of the X SS K4.6...
http://www.rcuniverse.com/forum/m_51...tm.htm#5161333