Somehow got a original boxed copy on Goozex back when that was worth a damn. Near mint condition, couldn't believe my luck. Got some other gems off there as well, although most didn't come in their original packaging. I would probably pay for a GOG version, as installation is long and tedious for only a 3 disc game.

The retail release has a locally checked serial number, and disc check. The one on DD and Steam, I have no idea.

But, you can always replace the exe with a cracked one, to get rid of the DRM before running the game for the first time.