Background: I am a computer science student and am currently employed as a programmer for a health institution. Recently we discovered the need for a project which is beyond a 1-man job (I am their only programmer), so my boss asked me to assist his working group to get a clear grasp of our requirements which we would then take to a software company.
We are now in a phase where we would like to get some offers from 2-3 software companies in order to estimate how much functionality we can get for how much money (which is both flexible within a certain margin) and I am starting to wonder how detailed we have to know what we want before doing so.
To put that to scale: We started with the idea that 'we need to get rid of this paper pile and enter stuff directly into a computer to speed up the process' and now have an 11-pages requirements elicitation document with a fairly precise description of what we would like to achieve in business-terms plus some non-functional requirements but no use case specifications or GUI mock-ups which show the changed business process.
Since the requirements elicitation document is usually a fairly large part of any software project, I wonder where to stop. Would you recommend me drafting some use cases to get an even more distinguished idea or would I just be doing the contractor's work?