you don't need to create a data model. IMO, it doesn't even help you coming up with a good architecture because it distracts you from OOD. You'll probably better off taking the business domain model and do some refinements and extensions here (if you think something is missing or inaccurate).
I am yet to start my assignment ( I downloaded in 2003 ), but never found time (or may be skill) to start it.
What I think is, people often use domain model, business model, object model interchangeably. If any of these models is going to help you refine what has been provided in the assignment and come up with the required class diagram, then you are free to do so.
Most likely, I will start my assignment in a weeks time. So hopefully, we can share more design information as we get along.
My suggestion is to construct domain model first(from the requirement analysis) then from there build object model and datamodel(object model and datamodel need not depends on each other, for any mismatches between these models can be addressed in OR mapping)