Try to describe your project goal in one sentence. This helps you to focus on the main goal of the project. What is the aim of your project. Is there a system that has to be explained or is it a range of services, is it a floor plan for visitors or is it for a safety manual, or perhaps both? Is there a solid idea for what you want or are the ideas more vague? Try to define your objectives and goals before the briefing.
Budget is always difficult. It's often hard to envision the final image the client expects and for a client it is hard to know what I will come up with. It has happened more than once that a client pushes me to do some extra after the project was actually finalized and the budget was reached. If you roughly let me in on a budget range I can tailor my services to give you the most benefit for your money. And if that's hard send me an image from my website that comes close to what you want.
Let me know how you schedule this project. It's the only way to integrate it into my planning and I will be able to tell if this time schedule is realistic or not. Is the illustration linked to a publication date or a public event. I don't want to know the ultimate deadline. Pin down the deadline a few days earlier. Never brief me the day prior to a three week holiday and have a colleague handle the feedback. It's always a fiasco.
Who are you trying to reach with this illustration? Is it a cutaway view for display at a marine museum or is it a technical cutaway for a product selling.
Good visual documentation is a must. If I have to create a cutaway view of a concert hall with the floor levels clearly visible it makes sense the client provides plans or scans. I can reject a project if documentation is not provided. Try to make a rough drawing. It really helps to find out what you want.