Well in an MP everyone automatically plays with the host's DLCs, so it is possible for you to play an up to date multiplayer if that is a concern.
On the other hand, yes. Way too many DLC. I don't even know how many of them are there! I have them all except Mare Nostrum, since I am an obssesive of completion (also I dont buy anything over 10$ ever other than the occasional base game), but Art of War and Cossacks are really the two must-haves, I think.
Are you aware of the radical change that is coming in October 11th? Institutions will substitute tech groups. That will come on the free patch, but the Rights of Man expansion comes with some reaaaaaally juicy stuff like CONSORTS, who can rule if the king dies before the heir comes of age. Looks like it will also be a must-have.