Where are you getting the PC from? I got mine from Chemical Guys and the kit came with the backing plate. I forget the size, but I know it fits the 5.5" pads with no issues.
You want an assortment of pads, since they all do different things. Just take a look at any of the good detailing product sites. They tell you exactly what they do (cut, finish, wax/seal etc). Get a bunch of different ones. I buy them in bulk because I'm not paying for the pad cleaner. That contraption looks good for pro detailers who detail many people's cars, but I can just buy new pads for me (they aren't that bad on the wallet). Plus, I don't actually use them THAT often, maybe twice a year.
'11 BMW E92 ///M3 - ZCP and DCT
'15 Ford F-250 - Lariat, 6.7 Powerstroke Turbo-diesel