Base game has about 150 hours of story, more if you really make the most out of it. Also includes the first 4 pieces of DLC for free.
First of all, the DLC sold on Steam are deluxe editions, which are way expensive (they cost 4 times as much and only add a few extras). The in-game store has them cheaper. You can also buy the Ultimate Edition (on Steam) or Complete Edition (on their site). In the case of Complete, it's like 60 bucks for game+ all DLC, and you can redeem it on an existing account to just get every DLC.
If you'd rather just buy DLC separatedly, here's a breakdown of them:
Issues 5-7: Side story missions, semi-relevant to main story, specially issue 7 is important. They each also include a new weapon.
Issue 8: Filler endgame grind, introduces wave-defense type scenarios to upgrade your skills.
Issues 9-11: Vital to the story, concludes the first season of the story, must have.
Issue 12: Adds dungeons for the endgame post Issue 11, exclusively group content.
Issue 13: adds missions to help fill a bit the final area of the game.
Issue 14: side story.
Issue 15: huge mission pack, really boosts the experience of the final zone in the game, since otherwise it's pretty barren of quests.