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.