You absolutely need the Codex. You need the rulebook, but you can pickup the mini-rulebook that comes with
AOBR really cheap on eBay. It has all the rules, but none of the modelling/background stuff that the big Rulebook has. You'd also need templates and dice.
Considering that the rules are due to be updated in the middle of next year, I'd pick up the mini one on eBay rather than spend money on the big one.
Of course, if you actually wanted to buy the
AOBR set, you'd also get all the 'quick start' stuff, which is a really useful tool when you're learning. It depends if you want the extra models or not...