A couple of things I think need mentioning:

- Diablo 1 does not use any DRM, just install and play from the disk. Because of this a used copycan be had on Amazon for about $5. Alternatively, you could look for an old version of the Battle Chest in stores (which would come with D1).
- The Hellfire expansion pack wasn't made by Blizzard but by Sierra, and it isn't considered a part of the original game.
- HD patches for D2 break the game's AI. They will spawn, but you will see them before they see you. It ends up taking most of the challenge out of the game.
- Diablo 2 is worth owning.
- If you aren't interested in realm play, buy an old used Battlechest off of Amazon. It should run you about $15, it will come with D1 and D2/LoD, and the CD keys (while being useless on Battle.net) will work to install the game and get you playing offline.