6 is a great age!
She will be a full adult which is awesome ;-)
Has she only been with her current owners a short time? (Just wondering because you mentioned she doesn't get along with their "current dog" so I'm guessing their pit bull has been there longer.)
Definitely get her established with a vet / bring her in for a check ASAP and, while you're waiting to bring her home, read around: Rescue, Foster & Adoption Information (General) - German Shepherd Dog Forums
and Welcome to the GSD/FAQ's for the first time owner - German Shepherd Dog Forums
and Health Issues - German Shepherd Dog Forums
and, well, just everywhere
Good luck! Will Diamond be your first GSD?