I've been in this kind of situation, but with less expertise on the client's side. My (successful) angle of attack was to review the documentation provided as

I've been in this kind of situation, but with less expertise on the client's side. My (successful) angle of attack was to review the documentation provided as a kind of "requirements dump" rather than a "this is how it should work" definition. I then applied my own knowledge and experience to generate a document that said "these are the features you asked for, here's how to provide them in a user-centric manner." As your client apparently has the IA and graphics person on their side, you may be more constrained. I'd still put forward some ideas, in a conversational manner. Two reasons for this. First, maybe I have a better idea than them. Second, it shows that I'm more than a code monkey (or a bunch of code monkeys calling itself a consultancy). The work product should include some idea of how you're going to attack the problem, and a sense of how long it'll take (fixed bids are for the devil), and prioritization based on client requirements and prerequisites that those requirements may have.