Hearthpwn

I don't play a lot of Wild, so I have no idea of the state of the meta there. Judging by the threads on Hearthpwn however, Big Priest is still a menace. As far as card packs go, if you wish to play Wild, the last expansion, Saviors of Uldum, adds multiple Highlander-cards that see play in Wild as well. Most notably, Zephrys the Great and Reno the Relicologist. I would not recommend buying anything from Witchwood, Boomsday or Rumble, as most cards in those sets see very little play in Wild, as far as I know.

If you have a desire to play Standard, it greatly depends on which style you like. If it's Control, and you like winning, Control Warrior is the still the beast to beat. Most of these cards come from Boomsday. Highlander Mage and Highlander Hunter are also strong picks, which would mean you want to invest in Saviors of Uldum. If you wish to play Aggro, Murloc Shaman and Murloc Pally, and maybe even Zoolock, have seen strong additions from Uldum as well. The very few Combo decks that are out there, rely on Mecha'thun or Chef Nomi, although Maly Druid has also seen a slight increase of play-rate.

As a general rule of thumb, I always tell people in similar situations to not buy packs from more than 2 expansions ago. This would mean Saviors of Uldum and Rise of Shadows. In this case, I would suggest that also. Specifically because you play Wild, which means fewer Legendaries actually see play. Additionally, Zilliax is a must-craft for most decks, while Zephrys the Great has also been named as a staple for any Highlander deck. Lastly, SoU will also give you a chance to get new Quests, but I'm unsure which ones see play in Wild.

EDIT: Just to clarify, I'd use your current dust, and the duplicate dust from your new packs, to craft the Legendaries you want from previous expansions.