What's the point of exploring the main game 100% and doing every sidequest if you're going to skip the DLC?
I've only played Overlord of the DLC that was released after the game, but I thought that was good and worth playing. Lair of the Shadow Broker is supposed to be the best DLC for the game, and you may as well play Arrival while you're at it, especially since that's the DLC that's supposed to bridge the gap between the two games the most.
And I think the Firewalker Pack was free? Otherwise you can skip that one, at least.