While I have been dialing my truck in since it came out, here are what I consider absolutely necessary to have a truck that can win against a decent field:
Slipper fix (pin it on slick tracks, add garodisks, get a clutch basket, or get the new center diff)
Swaybars
Adding weight to balance the car

Those three things will cost you less than $100

Here's what most people are finding to be very beneficial and you can add as you see fit/budget allows, but will probably be necessary to win at an upper level:
Chassis brace (Coyote carbon tube, exotec center chassis, etc. Just to keep belt tension more consistent)
Adding weight on rough tracks (not just to balance)
Saddle packs
RPM A-arms
Modded steering rack (like the rcshox.com ackbar)

As a final word, some might consider a chassis brace mandatory but from a durability standpoint, not a handling one.