For a lathe, make sure you get one with U-blocks or V-blocks. DO NOT get one with bearings.
If you use side by side packs, then you can attach the alligator clips from the motor to 3 cells in your pack and power your lathe. That should give you 3.6 volts which is fine.
Make sure you buy a lathe with a diamond bit. Carbide bits wear out over time, but a diamond is forever (sorry for the cliche
The trend for most brushes in stock motors is that they actually pick up performance on the second or third run. As long as your running stock, you can cut the comm every 2 runs if you're racing. And every 3-5 runs during practice. If you are racing 19 turn, then cut the comm every run for racing and 2-3 runs for practice.