Returning player

I want to get back into playing Warhammer 40k, but I'm not sure how to move forward. I last played around 13 years ago when I was in seventh grade. At the time, I had a Chaos Space Marine army, which should include around 2000 points worth of models, such as Ahriman, Thousand Sons, Berserkers, Terminators, Helldrake, Hellbrute, Demon Destroyer, CSM, Kranon, and Cultists.

I’ve noticed that Thousand Sons and World Eaters are now separate factions, and I’m wondering if it's possible to play them together in the current rules. If I have to choose just one of the factions, how can I paint my Terminators, CSM, and Cultists so that they all look cohesive? I’ve already painted my Helldrake in the Thousand Sons scheme, so I’d like to keep that as it is.