When I was looking at Document Management/Workflow Management (DMWM) solutions a few years ago, the two front runners for our purposes were Alfresco and SharePoint.
Alfresco is long on underlying functions, and, at the time, short on user interface, compared to MS SharePoint (2007 and 2010). It is open source with a commercial version with maintenance available, usually through local VARs.
You can download and install on your personal computer for testing purposes, or a nearby server. I've worked with it on Linux, Windows Server and Mac OS X. The client side is, of course, a web browser; the feature set is identical across browsers (FireFox 4 and later, IE8 and later, Safari, etc.). Installation is straightforward. Out of the box database options are limited to the free DBs, but you can likely install and manage other database drivers, JDBC being what it is.
Document management is extremely capable. Everything is a node; some nodes are directories or folders, some have files attached and some have web pages attached. Files are stored in the underlying file system while nodes and relationships between nodes are stored in the database. Creating new content types takes some work; I understand that more current versions of Alfresco make this a lot easier.
I ended up being very impressed with the document management capabilities. I have only played a bit with Activiti since it was incorporated into Alfresco, but I am excited to see Alfresco headed in that direction.
The basic, out of the box, community edition can be up and running very quickly in your organization. What will take time and effort is training users to understand data quality management, work flow systems and document management; this is true regardless of the platform you choose.