I wrote a rather long “guide” about all the expansions, look at the part “Rise of the Godslayer” → “Factions”, there I explain how the factions work, how to get the several gear sets and what faction suits your class.
As you might notice: There is no “perfect” faction for any class, if you want all items (weapons, rings, necklace, armor, potions) you will need to switch a faction at one point and work your way up at several different ones. That’s probably the reason why there is no explanation about this in the game though I agree that the faction system could be explained a little more in detail in the ingame help.