When to use a Flannel buffing wheel vs. a UBM buffing wheel
In most cases, you'll want to use a flannel buffing wheel when you have surfaces that you may need to work around, such as valve stems, buckles, brackets, etc. as the softer nature of the wheel will work its way around those obstacles/objects.
UBM Airway buffing wheels are more often used on flatter, simpler surfaces where working around things are less of a concern, such as diamond plate.
What other items do you need for metal polishing?
Masking tape is an absolute necessity when high speed polishing.
Makita Random Orbital Sander - Most popular sander for metal polishers