Hi Johannes,
It is indeed possible to use LDAP group permissions with SVN when using it with Apache (which is how I've always run it).
http://gentoo-wiki.com/HOWTO_Apache2_with_subversion_SVN_and_DAV
http://blogs.open.collab.net/svn/2007/03/subversion_ldap.html

I'd be happy to help with that.

Running SVN would give us access to the wonderful SVN::Notify, which I've attached a screenshot of.

 
And just as a reassurance for the IEM option, I've found SVN to be pretty light on resource use.  There are only a few developers committing on any single day here.

Cheers
Luke