you can legally own a .iso of a game that you own a physical (fully licenced) copy, you just cannot distribute it. So downloading via bittorrent would be illegal. Might be worth checking if you can download it via http or getttin a trial version that needs a legit cd key to unlock. I think the hitman series did that, they released the full game to download, but you needed a code to unlock past the first level.
Hope that is food for thought