No, you won't have to be online while playing it, and you'll still be able to do achievements. I'm sure of it because I've been using Steam for months.
Achievements are completely useless, yes. I hate those.
Haven't been minding Steam for quite a while though, it doesn't bother me at all. I think you'll be surprised.
Late to the party, but I'd like to add that I hope I can't just max EVERYTHING and the game will actually be challenging, you know? I plan on trying hardcore mode the first time through.