Good practice in brief-writing
Before writing the brief you should determine whether or not your organisation has commissioned work of this sort before and consider using a past brief as a model. Look at design briefs issued by a range of different agencies and talk to other clients. This can give clues about how interest in a project has been promoted in the past and what kinds of design practitioners respond to them.
Your brief has to emphasise what is important: it cannot simply be a long list of requirements, nor is it good practice to ask for an unreasonable amount of work for the budget available. It needs to put forward a viable proposition to potential partners and demonstrate that the client knows the value of what they are asking for.
You will need to adjust the scope of work to fit your budget before finalising the brief. Leaving the cost for the tenderers to determine can be a good way of getting quotes to determine the final budget, but it could come in much higher than expected and runs the risk of producing tenders with substantially variant prices which are then difficult to compare. In many cases, it may also be wise to retain a contingency sum to allow for refinements of the scope during the masterplanning process.
As well as setting out formal requirements, the brief is often your main opportunity as a client to promote or market the project as an opportunity to potential partners. Highlighting aspirations for quality, and a commitment to design as a process, is a good way to demonstrate intent.