If you are going to be connected to the net I would strongly advise setting up your firewall. Most distros will come with iptables turned on (firewalled) by default. You do need to tune it for your specific needs. Running without a firewall is asking for trouble. You can run over to sites like sheilds up (https://www.grc.com/x/ne.dll?bh0bkyd2
) to see how exposed you are to the net.
While installing a virus scanner is not necessary right now, it will be at some point. ClamAV is pretty good but rather than install it I just run over to trend micro's site once a month (or once a few months) and sweep the entire system. You will be amazed at the number of windows viri(harmless on linux) one picks up.