If you don't mind used or refurbished, you could look at a 24-70 version 1, and a 70-200 f/4 IS. It would be at the peak of your $2K, but still possible. The only lens I use for candid portrait work is the 70-200 2.8 ii. It is a beast, heavy enough actually that my wife doesn't even carry it when we have a wedding or portrait shoot. If you want the optics despite the weight, I have absolutely nothing against the "beast" tele, 70-200ii.
Cheers,
-Tabor
P.S. If budget is an issue, I would go for primes as others have suggested. If you picked up a 35, 85, and 135, you would be set for just about any situation that should come your way.