Have you looked at Sharepoint which is basically what you are describing?
Why not just use subversion with one of the many clients that might suit your needs out of the box?
Subversion does sound like the best bet here, just train the users that have to manage the documents in how to use TortoiseSVN and then have your document storage be a sub directory of your project.
If you are building it yourself and using C# and SVN then sharpSVN looks like a good solution. With full access to the SVN Client API you should be able to happily be able to integrate the web application to a SVN backend. Enforcing security by SVN properties or some other meta data store.
Some of the problems I can see are:
- you that if you have multiple sites it maybe hard to get different SVN Repos to play nice when they have to sync up (if they have to).
- SVN doesn't do much more than store binary blobs, so it will only version the objects, and not be able to give you diffs of them. This may lead to a very large SVN database.
- It is also currently not possible to purge a file from a repo, which will also lead to large SVN repos.
- If different people can revise documents, and expect them to be different "branches" of the document, then it may be better to use a distributed source control model where such things are easier.
Although it would be nice to re-use something like Subversion on the server, it does make application roll-out more complicated. If you're happy with this then sharpSVN (as suggested by Andrew Cox) seems like a good fit.
If you're less worried about some of the version control aspects of Subversion (diffs and branches) then I'd be tempted to knock together a quick document management module to your existing application.
I would use Lucene to handle the tricky stuff - indexing and full text search.
If I were you I would investigate Sharepoint, or at least Sharepoint Portal services. This is precisely the scenario the solution addresses
We opted to build our own domain model, and store documents in a SQL Server 2008 database. So far this has worked great for us.