Picked up the 3 pack of games on Steam (only $8.75 this weekend for all 3) Technically it's just Overlord 1, the expansion for it, and Overlord 2, but so far, Raising Hell seems to be quite a hefty expansion. I only went through the first Raising Hell section, but it seems to me, you wouldn't even realize where the expansion parts were if you hadn't played the game without the expansion first.
Buying the three pack also means that Raising Hell is automatically added to Overlord 1, but I'd still suggest it. There just may be some areas where the puzzle elements are steeply more difficult because of the Raising Hell content. And yes, Overlord is in essence a puzzle game, but not in the Tetris style puzzle-arcade game. I still haven't beaten the base game, but that's more because I didn't know how, had to look it up, but I think I may go through the Raising Hell stuff first. Mostly, I can't wait to start on Overlord 2, if I can even run it, I barely meet minimum requirements I think, I really need to upgrade my PC, but it's still doing fine for now on most of the games I actually want to play.