I learned as a technical writer to:
Write for a specific target audience (e.g. CEOs, or IT people, or end-users, or developers)
Include all/only the information which that specific audience needs to do whatever it is that specific audience does (e.g. decide whether there's a business need for it, or decide whether it's easy to install and support, or decide whether it's easy to use, or learn how to customize it).
Start with a table of contents, to define what topics shouldn't be included (and which topics shouldn't be included and which should instead be links to other documents)
Therefore I find difficult to answer your question: because you haven't said who you'd be writing the proposal for, and because I don't know why these people would be reading your proposal (i.e. what are these people trying to do, and why).
Remember that people are probably trying to solve some problem they have, so it helps if you understand what their problem is.