You need ALL hub carriers, the rears, the fronts and the front uprights. Not only are they good for durability, but they also have bushings for the uprights. The stock you can bind the hubs by torquing the hubs screws too tight. The aluminum ones make the steering more precise and with less friction.
Do not get aluminum arms, or bumpers. I've seen a bunch of broken aluminum bumpers at the track.
Shim your diffs. I believe I used 5 or 6 shims. You need to buy more with the kit.
In hot weather the HPI plastic gets very soft and I had my steering ball cups rip right out of the steering linkage, so get the aluminum setup.
Instead of using the stock ball cups, buy RPM cups, the stocks will pop or break easily.
As you run it more and more, sometimes you will hear clicking landing jumps. These are the ring gears skipping and the solution is to get the aluinum tranny cases, or shim the bevels, or both.
I have a low end pipe to wake up the 18SS engine and the ofna hyper 7 125cc tank to last 10+ minutes. I only desire the aluminum gearboxes and the aluminum steering links and that's it, then a new engine.
The 18SS engine has really weak compression, you should consider an OS .18 engine.