Which pads to get?
All of them; the proper question is how many of each?
Well, it depends. Normally I'd say, the finer the grit, the quicker you will use it up.
I'd suggest starting with 3-5 x 500 grit, 4 - 8 x 1000 grit, 5 x 1500 grit, 10 x 2000 grit and 10 - 20 x 4000 grit,
BUT, in truth, it depends on how rev/speed dominant you are and how much oil you usually face.
If you're speed dominant, you'll need more 1000, 1500 & 2000 grit than you will 4000. If you face more light oil and are rev dominant, you'll need more 4000 grit and more polish.
If you're unsure, start with 3 x 500, 5 x 1000, 5 x 1500, 5 x 2000 and 10 x 4000.
The finer the grit, the shorter (less uses) time you'll be able to use them.
For instance, I get 3 - 5 full uses from a 4000 grit pad before it begins to polish balls (4000 grit should always leave balls dull looking.) 2000 grit pads last 4 - 7 uses, roughly for me. 1000 grit last 7 - 10 uses, roughly.
Save the used pads to apply compounds and polishes. They're great for that.