And v13.0 is based on Debian Wheezy (aka Debian 7). So often you find good answers to TurnKey questions by consulting the Debian docs and/or google.

As for the desktop, personally I wouldn't do it. I like a desktop on my everyday usage PCs but I like to keep my server light weight. TurnKey appliances also have Webmin pre-installed which gives you a GUI to control most aspects of the server. Most desktop software that might be useful can usually be run remotely (i.e. from your existing desktop and connect via SSH or similar). Also modern desktops like Gnome or KDE are beasts and use a ton of system resources; not something that you would generally want on a server IMO.

Personally I would recommend that you get comfortable with the tools available by default because IMO a desktop doesn't really add anything to the user experience. Unless you use Webmin you will still often need to edit text files and use commandline to complete some tasks (both of which can be fairly easily be done either via CLI or remotely). Unlike Windows, Linux servers are generally designed to be controlled via commandline so there are limited GUI tools available...

However, we are all about freedom and if I haven't convinced you already and you still want to add a desktop then you can! :) I am not familiar with doing that TBH (as I've never done it) but I'm pretty handy with both Debian and google! A quick google turned up the Debian Gnome wiki page which suggests a few different option. I wouldn't personally use tasksel or install the 'gnome' package as it will drag in a ton of extra stuff that unless you also want to use your server as an everyday machine is overkill (IMO anyway).

So according to the wiki page you could install 'gnome-desktop-environment' which would probably be the closest to a complete but not too bloated Gnome desktop. Although I would probably install 'gnome-core' first and then just install the specific bits you need...

Unless you go the tasksel route, you can install packages with apt like this:

apt-get update #updates local list of available packages
apt-get install <package-name>

If you are not totally sure of what the package is called that you want to install then you can do a Debian package search (note default search is not for Jessie, you will need to search for Wheezy packages on the website) and/or search via the commandline with apt:

apt-cache search <keyword>

Sometimes that gives too many results so you can reduce them by using grep and a second keyword:

apt-cache search <keyword1> | grep <keyword2>