Sorry but I don't know the answer to your first question, but in answer to your second I've gone with the standard Boltgun Metal. I don't care what people say, to me and my foes they look pretty intimidating. It also ensures they're quick'n'easy to paint.
Third question: I think that posing all your models the same would make for quite a monotonous army. They need variety, even for an army that (fluffwise) is quite unified and robotic. Having said that, Necron fluff dictates that Necrons frequently succumb to misfires and stuff, like random electrical discharges in their legs, so they're not completely samey. In my army I have half of them pointing their guns at the enemy, and the other half holding their guns to the side, but all in varied positions, for instance one may have their gun raised up, and the other down low.
Hope that helps