Hi, and welcome to Dakka!
Apart from the rulebook (there's a free version, but it doesn't contain the matched play rules, which I recommend) and the codex, you may need the Index Chaos if you want to play any non-current models. There's also Chapter Approved 2017, which includes a few points updates as well as some other changes, but these changes can be quite easily found online and I wouldn't recommend getting it early on unless you really want to attend tournaments.
I do recommend reading the relevant errata though. They can be found here: https://www.warhammer-community.com/faqs/
. If you have any further rules questions, check out You Make Da Call here on Dakka, where rules questions are answered.
You can most definitely expand what you have into a larger army if you wish. Chaos Space Marines have a huge line of models, I recommend checking out the Tactics subforum here on Dakka for tips on what to get, and how to build a list of your models. There are also other Chaos models you can play, though they may require to buy further codices.