I'm just getting back into the hobby with a chaos force as well! Using the BA codex and making Night Lords Marines seems like a really good way to go. It'll allow you to collect what you feel like. Want some kickass Baal pred support, go for it! Want to throw in some daemon princes and play chaos over the weekend or in a campaign, why not? This opens up a lot of avenues for you, and allows you to play a really good jp army. Have fun!