Of changesets and updating work
Subversion and the Apache Subversion logo are registered trademarks of The Apache Software Foundation. Several companies (Collab Net, WANdisco, Visual SVN, elego, ...) pay or have payed the salaries of some full-time developers, but the software carries an Apache License which is fully compliant with the Debian Free Software Guidelines.
In other words, you are free to download, modify, and redistribute Subversion as you please; no permission from any company or any person is required. It is mature software, with strong compatibility guarantees.
The repository just stores a versioned directory tree — you may consider certain sub-trees to be projects, but Subversion doesn't treat them differently from any other sub-tree.
Also, the revision number should not be used as the publicly-visible release number of a particular project in the repository.
svnserve speaks a custom protocol, while mod_dav_svn uses Web DAV as its network protocol.
See chapter 6 in the Subversion book to learn more. The long answer: if you just want to access a repository, then you only need to build a Subversion client.
Writing programs to access the repository is similar to writing programs that use other filesystem APIs.
The main difference is that this particular filesystem doesn't lose data when written to; old tree states can be retrieved as easily the most recent state.
Server requirements depend on many factors, such as number of users, frequency of commits and other server related operations, repository size, and the load generated by custom repository hooks.