As a document for confirming with your client what work will be done, what you describe sounds like a workable approach. Your client won't likely understand E-R diagrams or other such important technical oriented design tools, they need to see a documentation of system function and results and pretty much any word processing type tool can be used to generate such a list.
I will start with a mind map in Curio to work it out for myself, it can then turn it into a list that can be polished in Pages.
Ask them what they need for "security". It something which may be simple, or it could be not so simple (though FileMaker does it well, I think). Some clients may expect a fair amount of "who can see, do what where".
As this is a pitch exercise, as they are currently doing it all on excel and word (sorry for swearing :) ), thanks for the idea.
i want it to be deliverable on any iDevice.