Here is some reading material that might help:
Gearing: Find a more experienced racer at the track and ask him for his gearing or "rollout". Copy that until you understand more.
If you switch to a more torque-based motor, use a bigger pinion... but only go up one or two teeth at at time. Use a relatively smaller pinion for a more rpm-based motor.
Brushes: Just use 766 brushes for now... and F brushes later when you need more power.
I think the following links answer most of your remaining questions.
I don't recommend turning your motor on a lathe after every run until you become a much better racer. Clean your motor out well after every 1-2 runs, and turn it on a lathe less often (like every 10 runs?) or so.
Motor maintenance is a funny thing... I think a lot of beginning racers spend too much time trying to get their motors super-fast... but their time at the track would be better-spent talking to their fellow racers, learning, and practicing.
Anyway, Hope it helps. Try hard to read... a lot... before you ask too many more questions. Pretty much everything you want to know is on rctech somewhere.