nofd1977- I think you oughta do a little research so you understand about the backing plates/pads/etc. A general understanding *before* you spend your money would be a very good idea.
I hate to say "go search" but almost anything that could possibly be discussed about the PC has been, and many times
Short version: The PC is designed as a finish sander; it's made to spin a single sheet of sandpaper *NOT* a big foam pad that's wet/heavy with product. Yeah, it'll do it, but it's just not optimal.
IME the larger pads are too big and heavy and result in too much friction. Once you apply a little pressure (in the inevitable attempt to make the machine behave more aggressively) the PC will bog down and just "jiggle", doing little of value.
With smaller pads you can apply more pressure before this happens and the PC just generally works better with smaller pads.
I only use 4" pads for serious correction via PC, well, I *might* use a larger pad for VERY gentle work but I hardly ever do that.
Note that almost *all* correction ends up being "serious correction" as removing marring via PC is invariably much tougher than people expect it to be.
I'd steer away from "kits" that include pads/etc. Learn about the thing, decide what you (most likely) *really* need, and just buy that. Get at least two of each pad because [stuff] happens. Very short version:
Get a small backing plate for 4" pads. Get some 4" orange light cutting pads and some 4" white/green polishing pads. Maybe get some larger polishing pads. If you have especially hard/soft paint the preceding recommendation should be changed to fit your situation, which is why it's hard to tell other people what they oughta buy.