If planning on a Mac, have a look at the official refurbished section of the Apple store website. Yes they are "used" but they still have the full 1 year Apple Care warranty. I got my iMac from there and I'm was very impressed with the savings that can be had.

As for viruses on the Mac, depends on how you classify a virus. If you're talking about; plug mac into your internet connection and within a very short space of time (30 seconds) and it will be infected with something, then no, that's not the case. If we are talking about downloading an app, running it, entering your password and it then installing various things which can log key presses, and do other things, yes, that is possible (typically called Trojan's) and there are a few, and Apple actually did create some protection system for one of them recently.
To be honest, if you're downloading stuff and entering your local password without thinking about what you are opening yourself up to then you're asking for trouble.

If I had to run AV software it'd probably be clamxav (think that's the name). I would not run something like Norton / Sophos. Had bad experiences with them in the past (clogging up the system, being too protective). They only really exist to prevent you from forwarding on viruses to windows based users, which could include your mac if you happen to have a Windows OS installed in some form on the Mac (dual boot / VM).

Bottom line, the choice of machine really depends on what you're going to do, macs are nice from a design viewpoint but they are just your normal intel based PC in a fancy white dress with a different OS on them. You can of course buy a mac and run windows and *nix on it if you like.