We wrote The White March Part I walkthrough using versions 2.02 and 2.03 of the game. We wrote the Part II walkthrough using versions 3.01 and 3.02. In both cases we used the "normal" difficulty setting.
The White March Part I DLC triggers near the beginning of Act II of the main campaign after completing the quest Never Far from the Queen. We'd recommend completing most if not all of Act II before venturing to the White March. If you head there right away, then you'll probably have some success but get dominated in the boss and bounty fights.
If you complete almost all of the main campaign before starting the DLCs, then you can play a scaled version where the battles will still be tough for you, but where the rewards won't change. If you play the DLCs before starting Act III of the main campaign, then you can play Act III scaled as well.
Along with the White March content in the DLCs, you'll also discover a new Dyrwood location called Cragholdt Bluffs. This is a high-level location without any scaling. You should probably be at least level 12 before heading there.
You can meet three new companions in the DLCs. In Part I, you can meet Zahua (a monk) in Stalwart Village, and the Devil of Caroc (a rogue) in Galvino's Workshop. In Part 2, you can meet Maneha (a barbarian) in Stalwart Village.
You'll find a dozen soulbound items in the DLCs. These items have to be bound to a particular character, but they gain bonuses as you complete tasks for them, and they're more than worthwhile once you've maxed them out.
The DLCs raise the level cap from 12 to 16.