Buying key help question

g'day, been playing alot of arma 2 and arma 2 ao with a bit of dayz, my bro has now gotten into it and being cheap tight asses i was wondering if there was a cheaper way for him to join, i have arma 2 and arma 2 ao on disc, if i buy a key, can i use the install disc i used when i first installed arma 2 and put in the key i bought or do i need a new copy of the game, also would i need 2 keys ie a key for arma 2 and arma 2 ao or just the one, its been ages since i installed arma 2 so i dont remember

secondly a question about these keys, where do these keys come from and will a bought key eventally get banned randomly or are they fully working with no issues? when i say banned i dont mean from hacking i mean from the fact that there non legit keys

you can even use a pirated copy from torrent with these keys, and you'll never know what has been done on the key you buy, maybe the person it got stolen from managed to use some detected hacks with delayed bans

who would sell a cd key that costs like 18$ for only 2$ if its legit?

Do you need an Key? If yes add me on Skype: omaexalo