What I need to give you a useful proposal.
Honestly, not very much. A short written brief, a real deadline, and a rough sense of the budget band you're working in. There's no need for a 40-page RFP, just a paragraph of context, a list of the things you really need, a list of the things that would be nice if there's room, and any prior work you'd like me to read first. If you'd rather just talk it through, the discovery call covers the same ground in about 30 minutes.