DayZ isn't worth buying now. The major changes that are going to be implemented over the course of this year will make or break the game and so far, most of these changes haven't been implemented yet, which means that in its current state, it's pretty much the same sorry state it was in last year at this time but with a lot more window dressing.
No, SA is complete shit. Just buy ArmA 2 combined ops. It is way better and you are constantly discovering new possibilities.