I don't think there's such a huge ERD. We are talking about 100-200 tables just the core.
In contrast you have the data base itself (you can just use reverse engineering to get the ERD of the tables you are interested in)
and a pretty good documentation, both general and for every single package (some of them even with an ERD).
For the reverse engineering you have plenty of tools, I would recommend 'Case Studio' and 'Power Designer'.
