I don't know why you couldn't find a testing release. The Debian download pages all seem to have links to the weekly testing builds clearly indicated. Following the links on debian.org about the various distros led me straight
here, for example.
You don't need to go through all the trouble to reinstall though. You can upgrade your existing system from stable to testing with a simple "apt-get dist-upgrade" command (after adding the appropriate testing sources to your /etc/apt/sources.list first). You can learn all about using apt
here; I suggest you make yourself familiar with the main parts of it before getting too deep into Debian. You'll doubtlessly be using it a lot.
Once you have your system set up, you should be able to install what you want with a simple "apt-get install xorg". It will probably uninstall the xfree86 stuff automatically when you do it, or you could uninstall it manually with apt before you begin. It will also run an interactive configuration script when you install it, so make sure you know what settings you need to have before you begin.
There are occasional problems with conflicts when you make big changes like this, so you may have to run the installation more than once, or run "apt-get -f install" to fix a broken installation. This should all be covered in the how-to I linked to above though.