Spare parts I would recommend:

Diff cups (or just upgrade them like you did...you WON'T break them again)

Spur Gears

I upgraded my brakes to metal disc and metal plug, like a $15 upgrade at ehobbies.com (go to the cen option parts)

Upgrade servos

FailSafe

Air Filter

Pipe

Ok, not all spares (upgrades!) but they will help!