Here's my take as I've build a few of these over the years. I used the corded 10" Ryobi Orbital Buffer from Home Depot. The key to longevity is ventilation for the motor. Drilling ventilation holes in the side of the bucket is key but the head of the polisher also needs ventilation. I added rubber feet to the bottom of the bucket to raise it off the floor to allow for ventilation. I also affixed a 10 pound free weight in the bottom as the bucket polishers tend to wander when plugged in. This solved that issue and also provided added clearance for motor ventilation. You should also make some sort of divider for the balls. If they collide during a cycle, there's a possibility they will get white collision/burn marks left behind by the cleaning solution and friction from rubbing together. Some argue this, but that was my experience. Here's a couple pics of the last generation I built before I decided to go all in and build a Gold Crown themed Diamond polisher knockoff to match my table, light and cue rack.
Good luck with the project!
The last polisher I'll build/use: