Best way to hasten leveling up to 50 is to check message boards in towns and accept ALL of the things on it. Throughout the story missions, you will be performing actions related to those things anyway (sinking ships, killing leaders and mercenaries, etc.), so why not get additional XP for it?
On top of that, do the free "additional feats" DLC for extra XP and gear as soon as you are the appropriate level for it, and none of them require other DLC's to complete.
As far as what to spec as, I recommend Warrior since there are some boss fights that force you into open combat. You could try Hunter also, but that's based on your preference. It's kind of disappointing that there are no stealth boss fights, but I guess AC is more of an action-packed button-masher rather than a stealth-based game nowadays.