Check the Steam store on steam? Should be a l4d2 demo there.
Unless they removed it.
This. Worked fine for me, gonna buy the game soon and start modding.
Sorry to disappoint you, but the SDK isn't out yet, you'll have to stick to the L4D1 SDK first.
My opinnion on Left 4 Dead 2 is that it's better than Left 4 Dead 1. It basically improves on almost everything. The only things I miss are the survivors from L4D 1 and perhaps some campaigns.
However this is one of the things I sort of dislike about Left 4 Dead 2. It's simply better. It's only improving on Left 4 Dead 1, but otherwise plays the same. There is simply no reason besides nostalgia and friends to go back to play Left 4 Dead 1! L4D2 is simply a L4D1 killer in my opinnion.