Advanced membership site with secure backend profiles & data management

Wondering if anyone has used WPMU, BuddyPress or a combination of them to create a site with allot of back end functionality? By that, I need to have advanced back end customization that will allow users to login and fill out fields of information and upload files...then be able to retrieve them, change them and export them at a later date.

Any expereince and insight would be hugely appreciate. Just in the initial stages and want to make sure WPMU could be a possible framework.

Thanks!