Everything suggested so far is great from the admin point of view, however your users are going to be a little left out. If you plan on doing any serious hosting you should install cPanel/WHM on the system. This software is the industry standard for hosting environments. I work as a system administrator and as a technical support rep and trust me, it makes like easier and makes your customers happy. Commonly, these servers run Debian or CentOS with the cPanel/WHM software installed. Unfortunately, it is commercial software and you have to pay for a license.

You can also look into some opensource control panels that will also suit your needs, such as I-MSCP (this is the control panel I use on the server hosting unixhub.net) or Host-in-a-box.