Do I need a Document Management System as well as Author-it

No you wouldn't need a Document Management System as well as Author-it. A Document Management System refers to the management of files. This is effectively a replacement for the file system for the specific types of files. The system does not concern itself with the contents of the files, or even what type of file they are. It performs functions like indexing for searching, version control, workflow, security, and access control. Typically nothing apart from the most basic information is known about the file/document.