How's the s-rank in this game?  Easy, Hard, Impossible?  I would
start a poll but I don't seem to have the ability in this forum.

From what I've seen, the Nightmare difficulty isn't too bad, and the audio logs are reasonably easy to find. 
 
It's the multiplayer achievements you're going to have trouble with.

The multiplayer is just about healthy enough right now for you to grab those achievements. Personally, I've played through most of the game on Hard, and there's no way I'd want to replay it again on Nightmare difficulty. There are no checkpoints on that level, and it's entirely possible to die if you don't know what's coming up.