The plastic spur is just so the mesh is quieter - no other benefits (I guess it's lighter too...). You do just run the normal hardened pinions on the plastic spur.
Tell me what the internal ratio of the 8ight is and I'll tell you a pinion to run - I can never find out what the gear ratio is though - so it's best if you just say.
The 1800KV buggy motor is 1512 sized - the same as the 2650 - smaller than the 1515 2200KV motor. The other 1800KV motor is a 1518 - so larger than the 2200KV. You'd want the 1512.
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