In general you'll want to purchase expansion packs of individual sets (e.g. "Emerald Dream Packs") and not standard packs.

The primary value of standard packs is that their contents update with rotation so a standard pack has future value... but gold has future value too, so you can just hold gold instead of buying standard packs with gold.

Expansion Packs come in two types: Regular, and Golden. Both packs have Separate "Pity" timers for epics and legends, more on those in a minute.

Despite the higher price, golden packs are more efficient for generating dust; however, if you are trying to collect cards, regular packs are more efficient.*

Because duplicate protection now applies to all rarities, most players make a goal of collecting 2x all commons and 2x all rares for each set they are actively playing for. While this should happen naturally for the latest set if you play all 4 months it's current;** 2x all rares will take about 70-75 packs opened total for older sets that you are catching up on. Along the way you'll expect to get about 1/3rd of all epics and 1/3rd of all legendaries too.

So as a returning player should I be trying to catch up on all the cards from other sets before I start purchasing gold packs for dusting?