1) Rear shock tower
Try getting the Kawahara or Skyline ones.
2) Fuel tank
It leaks (around the joint) easily especially in a collision.
3) Brake Pulley
Wears off easily. Kyosho has already come up with an aluminium pulley. Similarly Kawahara and 3 Racing have made option pulleys as well.
4) Pinions
Get the steel ones from Kyosho, Kawahara and KFactory. The original ones wear off real fast!
These are the only items require extra attention, other than that the belts and etc last pretty long for mine
Good luck!