most commen broken parts or good to have spares are

2 complete diffs
fully assembled transmission
complete set of arms
If your not running aluminun servo horns then spares of those
bearings of all sizes for your truck
shims
couple drive shafts
axle carrier

thats about it really. Everything else should just be checked regularly for maitence because its doubtfull to ever break unless you just dont check over your stuff after big races.