Now with the steam november promotion I wanted to take the opportunity to buy a dlc pack, do I need the expensive ultimate pack or does the empire pack already have all the dlcs that are mandatory for the gaming experience?...
Technically, all DLCs are optional. Certain DLCs are required to play for colonial customly designed nation, or using random new world, but other than that game is playable with no DLCs at all.
Most important DLC is Art of War, it adds a lot of stuff which is very useful, and not all of it is strictly related to war (though war is certainly very important aspect of the game). Other than that, priorities may be different depending on which nations, religions and regions you are going to play. BTW, there are quite a few "DLC tops" on youtube etc, where authors review of DLC features and provide their own order of priority.
Note that starting from base game and adding DLCs with features you want to try out is very easy, but if you have experience with playing full game, it can be hard to go backwards, as you will be missing a lot of tiny buttons here and there which you've used to use in the past.
