I'm unfamilar with ACreed. Are those add-ons on completely different games?
Completely different games.
I don't speak for everyone, but if I were you I'd skip the first one and just read up on the story online.
Very repetitive, tiring, boring. The difference b/w 1 and 2 is night and day, 2 was much more enjoyable. I would imagine they continued to improve, but I haven't played any except the first 2.