In my opinion as a huge fan of PoE 1, White March is by far the best part of the game, especially if you like dungeon crawling. It's better to play it between act 3 and 4 so you can actually use the cool loot and companions you get for the toughest parts of the game.

Also, I thought PoE 1 + DLC was way better than PoE 2 + DLC but I'm probably in the minority on that one.

I happen to be two years deep into a playthrough now, and the DLC seems very good. I would suggest you do it midway through your adventure, though, as I got mixed messages from the game's plot/NPCs about what was coming up, so I put the White March questline and some other tasks off till the end, thinking I wasn't at the last chapter and could visit them later. Then I realized I was at the end of the game, so had to rollback my save and I am doing those now, but it seems like I don't really stand to benefit much from it now, since I'm already overpowered, nor does the content feel as relevant. Good luck with the recovery.

It's definitely worth it, but if this is your first cRPG and you're on a budget I'd at least start the game first and see if you're liking it through the first act. You can add in the DLC at any time; it will work with an existing save game.