I bought arma 2 to play DayZ also. From what I understood you also need Operation Arrowhead in order to play DayZ. They say you'll be able to buy DayZ by itself soon, but they have been saying it for like a year. I bought Combined Operations at $30 from amazon but now it's at $60, don't know why.
EDIT: If you can get the stand alone version get it. Arma2 without DayZ is boring.