Hi guys you might not already know this but for anyone thinking about picking this game up Crusader Kings II is currently on sale on a 75% off (stealth?) on Steam thanks to it being ported over to linux. That means CK2 has the "steam play" tag on it which means it can be played on MAC, Linux and PC.
You will notice that most of the DLC is 75% off too apart from the lastest one, The Republic. Well you can get the steam version of that for 20% off here as long as you use this code.
Also don't worry about greenmangaming they are legit shop so you don't need to worry about having your Steam account banned or any crap like that.
If you are wondering what DLCs are worth it I would say ...
Crusader Kings II: The Republic ~ Allows you to play as a ship faring trader family.
Crusader Kings II: Sunset Invasion ~ Allows you to play as the aztecs, was released as a bit of a joke for 2012 year ending stuff so not historically accurate but still fun!
Crusader Kings II: Legacy of Rome ~ Allows you to play as Rome also you can "Reunite" countries merging them in to a massive country.
Crusader Kings II: Sword of Islam ~ Play as Muslims in a time where everyone is a jerk and trying to take your land away from you.
If you are a sucker and like pretty things the next thing I would suggest getting is the cosmetic stuff like shields and unit sprites and ruler portraits (so it does not look like a bunch of white westerners ruling your Muslim country for example) as they are super cheap right now on steam.
I would not purchase the music or the ruler designer. Not that these things are bad it just I find them not to be essential for enjoying the game and they do happen to bump up the price quite a bit too.
Hope this has been helpful! :)