First, I rather doubt that the Bachmann Big Hauler motor is 12v. More than likely 18 to 20v. I would suggest that you match you motor voltage to the voltage available from your power pack or less.
You won't need to make your gears, I buy my many of my gears from Stock drive in NY. I specify brass hubs with an acetal gear face. Boston Gear also makes good gears similar to Stock Drive.
I have my worms custom made, but Stock Drive has the necessary worms. Specify double thread worms and worm gears to
avoid surging on downgrades. I recommend 32 pitch gears and worms. The best overall gear ratio is 30:1 although 40:1 is a lot of fun
especially in switching and yard work.
Bachmann's biggest problem in their Plus drives (the one between the battery powered, first track powered and the current fifth generation)
was that the motor moved. The gear were fine, but the motor moved and started chewing on the gears. To fix the Plus units, simply run a hefty
wire tie strip down past the loco weight around the motor, back up past the weight again, thread the tie together as tightly as possible. That will allow
you a long life on your loco. If I can be of any help to, just ask.
Barry - BBT