Probably more horrors and another 1-2 Heavy support, soul grinder and/or Daemon Prince
Oh and if your new, just start getting games in at the points level you can field so you can understand how the army works better and how you like to play it, then you may see some insight in what to invest in. Daemons can be a bunch of very different armies with all the specialization. One could go heavy assault, or shooty or even they sludgy nurgle tally army. Can run speedy, slow a mix, there are lots of options. Can even run a Zilla type army will lots of monstrous nasties. You have the codex, right?
Will you be running against a bunch of different armies, or mostly the same? I would probably build my army differently based on that alone.