I'm gonna go ahead and disagree with everyone in this thread.
If you have the opportunity for a good deal and you'll enjoy repainting those used models, go for it!
I've been playing chaos since the early 90's and other than the travesty that was 3rd edition and the 3rd ed chaos codex I've always enjoyed my army. Granted I don't get to play much anymore, but I still sculpt, paint, convert, and buy stuff. Painting and converting is one of the best parts of the chaos army.
I think its pretty safe to buy any of the cult units they're not going anywhere. The nuts and bolts might change, but there will still be Khorne, Nurgle, Slaanesh, and Tzeentch cult troops. Same with obliterators, raptors, bikes, chosen, terminators, and possessed. In the worst case scenario you have to convert models that are no longer as good to be something else or horror of horrors, buy more models.
Chaos units rarely go out of the codex. I think the only unit I have that was removed was the original Blood Slaughterer, of which I have 3. BUT I can sub them in as Khornate themed obliterators in a heartbeat.
I have models stretching back to Rogue trader and I still use a lot of them. Chaos doesn't go out of fashion so much as get more varied and chaotic.
What I wouldn't do is go overly heavy in any one direction, unless you really like the models. If you're going to building an army in any way based on effectiveness than try to hedge your bets in the meantime before the new Chaos book comes out. Build only one or two of each unit or vehicle that you enjoy.
I doubt you'll even have the time to get that far before the book and the starter set are out and give you more concrete direction.
And feel free to PM me if you want to talk Chaos.