You definitely want variable speed and the larger size as the smaller ones, say six inches or less, can build up too much speed meaning heat which can bugger your paint. I've had a Snap-On brand from when I had a restoration shop 17 years ago and it's been super, needing only a bearing replaced in all that time and use. It's best feature was a dial for variable speed limit, so you could vary the speed with the trigger up to what the dial was set at. Whenever I got a new detailer that kept him out of trouble!
As for pads and compounds, stop by your local auto body supplier and you can get the "good stuff" there like 3M machine compounds and any variety of finishes. We used Meguairs commercial products in my shops along with some specialty items available from the supplier, such as swirl remover for use on really dark finishes like black. You'll get what you need there as well as proper advice on what to get and how to use. Just remember to go slow, watch the corners as heat builds fast there, pop the hood when hitting it and don't dig into panel gaps.
DO NOT USE ON CLADDING! It's a pain, but hand application can't be beaten on polpropylene parts.