You will need a custom
# command line
sudo -e /etc/X11/xorg.conf
gksu gedit /etc/X11/xorg.conf
xorg.conf is divided into sections, and you only need a few sections.
You may not need more then a Monitor section
Option "Enable" "false"
Option "Enable" "true"
If you need more then that ...
In terms of run levels, Ubuntu uses Upstart, and to some extent the idea of runlevels is depreciated. You will need to edit your Upstart scripts so that X does not start in runlevel 2 and append "2" or "3" to your grub linux (kernel) line.
Upstart is not too bad to learn, see Upstart How To to get started.
That should get you started, post back if you get stuck.