Unfortunately, the battleforce, while a nice start, isn't going to be that competetive without some heavy support.
I love Khorne Berserkers, but I love them even more assaulting out of a land raider.
2 of the best units in
CSM, a codex that's struggling to keep up with 5e, are the Daemon Prince and Obliterators. The
DP box can be found for under $20 on ebay, and oblits, used and poorly painted, can probably be snagged for around $10-12 if you're a good bargain hunter.
The real issue is that the Possessed are really inefficient points wise. I converted mine into noise marines using the emperor's children sonic weapons upgrade pack, but that was before the price hike this summer, and now it's like $22 for 5 guns, so not that financially sound...
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat440339a&prodId=prod1400035
The battleforce may not be the best choice for an inexpensive and effective chaos army - but one of the great things about
CSM is the fact that you can run another space marine army as a chaos army with some creative painting/modeling. So Ebay can really come through for you by providing you with the bits you need.
The advice that people gave me when I got started, and really holds true, is figure out what you REALLY want to play, and then zero in on those models. It will take you a lot longer to shop for, but in the end you'll be much happier with the result.
[edit: Wow I feel like I jerk, if you've already bought that battleforce nothing I said was helpful, in which case, ignore me]