Here are my suggestions to your questions:
I say go with #2 steel as an all around waterfowl load. I have dropped geese with this load but would prefer to use BB or Hevishot if goose hunting only.
My belief on decoys is, the more you have the better. :-D One thing you need to do with your dekes if you are just starting out is stick with mallards for the dabbler ducks. Any dabbler will come in to mallard decoys. It is nice to have different species in your spread, but if your just starting out, go with the basics.
Do yourself a favor and don't get cheap Chinese decoys. If you are going to duck hunt a while get good quality dekes. It is worth the investment.
One more thing, go cheap on your calls. Nothing sounds worse than a $5 duck call, or the ones you get free with a magazine subscription. I use a Haydel and these are decent and long lasting.
I hope this helps.
Good luck and get a bunch this fall.