The versions of SQL 2000 is the same for GW, Domino, or Exchange ...
I would stick with MSDE as it is free and can easily support a few hundred users.
You might look to Microsoft for a comparison of the editions
You could in theory have the BES be a DB server for other services but it isn't recommended due to the built in governor of MSDE.
From a database structure standpoint MSDE is the same as SQL 2000 Standard / Enterprise.