The character system is much simpler in MMXII. Instead of having skill requirements for each class (that might take you forever to achieve), you just have class requirements.
I'm with Claudius that you could go with the CE or MMXII. I like the game mechanics better in the CE, but MMXII has the better interface. If you really want to play in widescreen, then MMXII is your only option.
I'm not sure if there's a big co-op community for Dungeon Lords. Co-op hasn't really worked well in any version of the game.
Sir Edmund: "Should you obey the lord who asks you to put a village of innocents to the torch? Is that chivalrous? Is it noble?"
Me: "It's a great way to get promoted, I know that much."