So the installer customize the host and domain also it can add one user-account.
One thing is left. The keyboard-layout.

One is stored in /etc/X11/xorg.conf the other in gnome somewhere.
What will be the best way to add a keyboard-layout option to the installer?

for xorg.conf we can use a sed-cmd.
where is the layout in gnome stored and how to change that?

Maybe here: ~/.gconf/desktop/gnome/peripherals/keyboard/layout/%gconf.xml

The GNOME keyboard layout defaults to the xorg.conf keyboard layout, so if you change the xorg.conf setting during installation, everything will be fine.