Hi:

Graphical use cases are for to-be design of relatively finite systems.   Text based use cases are for to-be design of even smaller scale systems.  Your situation seems to require as-is modeling for (maybe?) larger scale integration. 

Also, maybe consider the need for business process reengineering, as the major benefit of larger scale automation efforts often revolve around simplification of existing business processes.  Use cases are not a business process reengineering tool. 

Tony

Our organisation is following a similar path, with the added variation that some projects could be aimed toward procurement of new 3rd party software.

The business analyst role here keeps use cases fairly simple (or 'black box'), concentrating on what the system does, not how it does it.  In that respect then, our use cases would be just the same regardless of whether the project was aimed at procurement, reuse of existing assets, or new development.

The next stage is where the different project types diverge.  For a procurement, we would send black box use cases to potential suppliers.  This level of use case gives suppliers a bit of freedom to gear their products toward a viable solution.  For reuse of existing assets or new development, our systems analysts and developers would analyse the black box use cases and begin to move into the solution domain, looking at how the system will provide the required functionality.