Your meat of your Brownfield Grant Proposal is to be 18-pages, period (Follow the Guidelines to the T, on formatting and number of pages, font, etc.). However, you can have some attachments and letters of support (as attachments) are encouraged. Ask for letters of support for other organizations and make sure that they are specific in how they are going to support your project. A boilerplate support letter is easy to get from just about anybody, but they need to describe what the organization will do related to the proposal. In other words, a letter of support from the Albanian Support Group for the Advancement of Paperwork means nothing if the group is not an integral part of the plan. This means homework, and homework that should be done now!
Good Luck!