Chris,
The earlier in the season you go (late October to mid November), the better the gunning will be. This is the time that the birds from the north are starting to show up and are not really accustomed to seeing boats and decoys together. Later in the season you will start to see eiders giving boats a wide berth. They are catching on! That said, eider can be some of the wariest birds I have hunted. They will often swing wide and give the boat/set up a couple passes before deciding to settle in or fly on. They are smart birds.
For decoys, I would say you need at least 2 dozen birds - either sleds or full bodies, or a collection of both. Sleds are great for pulling birds in from long distances. On the water, you will see your sleds from a half mile away, while the full-bodied decoys tend to get lost. If you search the forum for "eider sleds" you may come across a thread or two on construction. I can post some pics of my sleds that I built last year.
What do you have for a boat?
Nate