[Parks]

You are allowed to have whatever motors you want in your possession.

You can buy twenty drill motors - except only two could be on your bot at one time. It would probably be a good idea to buy extras.

The numbers in the transmission are the clutch mechanism. They stop power being transferred to the output shaft depending on how much resistance the shaft is experiencing.
Mess around with the different settings and notice the difference in when your motors will stop working. Ideally you want to set these on max (the drill symbol) or better yet to block out the whole mechanism.