In my opinion the only thing you should get is ArmA 2 + Operation Arrowhead, as the vanilla ARMA 2 game requires a beastly machine to run, but the OA engine runs much better, and you canb play all the arma 2 missions inside OA if you own arma 2.
I have arma 2 and oa on steam, I right click operation arrowhead and select "launch arma 2 combined ops" which basically launches all the games and missions in one (if I own them). Since I only have arma 2 and oa, I can't play the british armed forces missions or private military company missons, but I can use the content like guns and vehicles in the armoury or multiplayer or co-op.
Depending on the difficulty, you can switch between first and third person, or only first person.