I think if I was in the market for an iMac I too would look to last years model. Apple obsession with thin is totally out of freaking control. The more they go this route the less user serviceable these things become. Hell even the 2011 model isn't that serviceable.
Personally I am a Mac Mini guy... I don't like all in one systems. My current (2011) model Mac Mini server with the i7 quad gives me at least another two years before I need to be concerned with upgrading. I have a Thunderbolt 2TB drive (which is very fast). I also have a Blu-Ray/DVD/CD connected with a very high end display... I like the flexibility, but in all honesty the price for the iMac refurb just can't be beat even going with the Mac Mini. Unless you already have a very nice display, mouse and keyboard, otherwise for someone new coming in, the iMac is probably the best way to go.
While this years newer Mac Mini's have great speaks and now the mid level brings quad to the table. It just isn't enough for me to upgrade, which is why I went high end last time so I'd future proof myself having to upgrade.
I use Bitdefender which is free for virus scans. It isn't pretty, doesn't run in the background and because I am a safe user I run it about every 2 weeks to scan. They do update the virus signature on it daily. I have also tried Avast Free for Mac it works great too and does auto update and run in the background, but at the time I tried it, it really slowed down my SSD drive on my Mac Mini, so unless that has been fixed I can't recommend it. Norton's I hated it on Windows and I would never consider it on a Mac.