Get it from http://developer.apple.com. You will not be able to submit apps to any stores using this XCode version, so turn away if that is something you might want to do.
In XCode's Preferences > Downloads you can install command line tools.
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sudo xcode-select -switch /Applications/Xcode45-DP1.app/Contents/Developer
Visit http://xquartz.macosforge.org/trac/wiki and download and install 2.7.2+.
You will need to fix the symlink it makes:
ln -s /opt/X11 /usr/X11
brew update
brew upgrade
Try out everything one by one and when one doesn't work brew remove it and then reinstall it. As of this writing, I am using the same brew I already had installed (upgrading to Mountain Lion doesn't remove your brews and they should be able to continue working normally).
After installing mountain lion on a fresh machine which didn't have homebrew installed I failed installing svn. It was dependent on serf but I couldn't get it to install serf. In the end I got it to work by copying over my serf installation from a Lion install and then linking it. Since then everything else has worked fine including installing mysql.
Thanks for your post, it got me on the right road.