MW 5 hasn't, as far as I'm aware, gotten a physical release on any platform.
As far as getting the most bang for your buck, I'm not sure if it's still available but at one time there was a bundle of MW5: Mercs with the Heroes of the Inner Sphere and Legend of the Kestrel Lancers DLCs. I'd get that to begin with, then pick up the other DLCs (Call to Arms, Rise of Rasalhague, Dragon's Gambit, and Solaris Showdown) after you decide you want to commit to the game.