Honestly, I think your best bet isn't an Eclipse plugin at all, but Enterprise Architect by Sparx Systems. Its cost is reasonable ($200-300 US for the Professional version), and it can reverse engineer an existing code base into UML. I've used it on several projects, and have found it invaluable whenever I needed the 20,000-foot overview on an existing project.
Also, I haven't used its UML capabilities extensively, but MyEclipse (for $65 US) claims to have reverse-engineering capabilities as well.