epg is the svn maintainer, but I keep an eye on it too.
Right now, subversion-base gets you not only svn but also svnserve.
That's really the core program. www/ap2-subversion depends on
subversion-base and apache, and gets you the dav module.
devel/subversion is the kitchen sink, depending on base, the apache
module, and 4 language modules. There's no reason to install it
unless
you want everything (which clearly you don't!).
What subset are you trying to install that you are having trouble
with?
If all you want is the apache module, just installl www/ap2-subversion
and you should be all set.