Opening Moves for Jackson Howard should be a high priority. He goes into every corp deck because he fixes the issue of agenda flood.
The deluxe boxes will never rotate out so these should also be high priority. Each deluxe features one runner faction and one corp faction (except Data and Destiny which is NBN and introduce 3 new runner mini-factions).
Kala Ghoda is the newest data pack and Business First comes out on the 10th (in the USA, might be different if you're located elsewhere). I would recommend purchasing both and staying up to date with the game.
Once all that is done you'll have a very good idea of what factions you like best. Then you can look at what cards are in each of the past data packs for cards that will help you out and pick and choose at your leisure.