Alfresco stores the file content in the File system and not in the Database and DB has only index of those documents. If your users have 100G documents its most likely that the file system will grow. That makes Alfresco scale better than other proprietary systems. So there's no need to worry about the years and the count of docs in Alfresco, it scales very well.
Does Alfresco create a new file/directory system structure, or can it work with an existing one? If it needs to create a new file/directory structure, would it copy over the content from existing folder to the new folder?
