Lee,
are you planning to hook this system up to the internet? If not, you don't need anti-virus software. If you are, any of the 3 listed is a decent choice (Norton, McAfee, AVG). I've been using AVG for about 18 months now, and haven't noticed any problems. I'm pretty careful about opening email attachments, what software I download, etc. It does spend 30 seconds or so when you first boot up checking for updates, but I can live with that.
For a computer that probably keeps business records and truly vital information, I would be very leery of hooking it up to the internet unless you need to. If you do, make sure you keep GOOD backups (no anti-virus is fool proof, and can you really afford to lose what you have stored on that hard drive?)