You can also configure your network via /etc/init.d - scripts. See here:
https://dijks.wordpress.com/2012/07/06/how-to-disable-network-manager-in-ubuntu-12-04-precise/
AFAIK the network manager does not integrate into all desktop environments.
As for your idea about a media center I would simply remove all programs not
used (e.g. office, mail, etc). Then use a basic desktop environment such as
XFCE or LXDE. You can add / start then XBMC via the autostart function inside
the used desktop environment. I did so with Sabayon, XFCE and XBMC a while
ago. Your login manager might be configured for automatic login for the
corresponding user for simplicity.
If you need to do webbrowsing you simply could exit XBMC and start the
browser of your choice.
