1) a full set of ballbearings, and
2) some good tyres eg HPI X-patterns
the M03 doesn't *need* any options to make it go.
Build it up stock with just bearings and try it first.
If it rolls on cornering your car is too high. Limit the rideheight/droop by putting spacers inside the shockers - Just run as low a rideheight as you are able to. (This does a better job than the swaybar kit.)
If you run on a large track with long straights, the Toe-In Rear Blocks improve straight-line high-speed stability. This may or may not suit your driving style.
We race SportTuned motors here, they're nicely suited to the Mini. Good turn of speed, much improved over kit RS540SH and the Mini is quick. Guys have tried hotter motors in it; although it might look faster on the straights but lap times do not improve.
Originally posted by utieh
servo saver... the stock one has a lot of play
Cut some tiny slivers of PVC electrical tape to pad where the C-thingy's jaws bite into. Reassemble onto servo then tie a small cable tie around it on the outside.
But even without doing this, the little bit of slop in there doesn't cause any noticeable directional uncertainty anyway. Just run the thing stock first, then make up your own mind.