I had ordered the first four codices from the Fifth edition 40K from GCMinis. These are Space Marine, Imperial Guard, Tyranid, and Space Wolves. These should cover the basic armies I can actually field. I just got them on Monday so I haven't really had enough time to really go through them more than a cursory amount.
Now that I’m getting back into 40K and have the codices and the Assault on Black Reach rules, I can start looking at the gaps I need to fill. I should be able to field Space Marines or Tyranids without much effort. Orks were the last codex published before the new rules so they might be a while before the upgrade. I still want to get the Apocalypse rules as well.