As long as you are sure you have nav, you just need a nav disk.

You can buy a new 2012 DVD (actually there is 1 for East USA and 1 for West USA). You could also buy a used one on eBay or elsewhere (just make sure the seller is reputable and you're getting a OEM version and not a copy) for previous years.

The 2012 hasn't changed much since the 2007 version. It's a little better and has lots of POI (points of interest) updates.