MobileUserX gave you some good advice, 4PSA made a nice advertisment, so there you go... welcome to the forums
To chip in: viruses are not something you should be afraid of for your linux server. It would be nice to have some anti-virus software so that you clients don't get hit by all sorts of viruses that get sent to them by mail, that's true.
There are solutions that work with qmail (that's what Plesk uses for mail), but you have to install that yourself.
In any case, since you are asking the questions like you just have, my strong advice to you would be to install a linux system at home and play with it. Get to know it. When you can truly admin that, without using the GUI, install Plesk on top of it. Learn how it changes the administration aspect of things. At the same time, read the Plesk forums, WHT and this forum, learn.
If that's not the path you'd like to take, IMHO hire an admin or drop the idea of running a server...