Tanks are part of regular MP. With ArmA 3 coming out soon, there will be less noob DayZ players ruining the game. You buy the game for the ability to mod. ACE mod is just great, but there's a ton of other mods out there that blow your mind (enhanced AI, ballistics, sound effects, vehicles, dress and uniform). If you buy Combined Operations, you'll get both the original game (based off of Eastern Europe) and the Operation Arrowhead expansion pack (based off of Afghanistan), which is a definite plus.
I've played maps which were essentially remodels of the Kandahar Valley with my group.
But I will essentially be a newb
Is there an upgrade system also? So if I come in, I will get only be able to 'get' a crappy tank first?
No idea how MP works in ARMA