Hi, in general there is only one part that you must have and that is extra t-bars (I generally have atleast 3 when going on racemeetings) if you don't crash REALLY hard
You don't need to actually break it to replace it, as it says in the set-up book you should replace it when you notice the corners turning white or if you start to see cracks form.
So basicaly here is a list that you might concider
Things you must have:
Fiberglass T-Bar 1.5mm - Wide #371180 (or the other one if that is what you are using on the tracks you run at) As rx7ttlm mentioned, it is nice to have one complete T-bar assembled
Things that are good to have:
Front Wheel Axle #375220 (You may not need this but it might get bent if you touch somthing in a bad angle)
Steering Block - Right#372210 Left#372220 (If the front axle doesn't bend then this will probably break if you hit somthing in a bad way)
E-Clip 1.9 #965019
Spur Gear - 96T/64P #375896 (or whatever size you run)
D-Lock Differential Plate (2)#375080
Things you might need in case of a really bad crash:
Suspension Arm - Front Lower - Right#372110 Left#372120
King Pin #372280
Rear Pod Lower Plate #371140
Rear Brace #373080
Ride Height Adjuster Set #372070
Oh in worst case...a new chassie....