I totally agree with Chip. Whatever you decide you should make use of child themes for sure. As long as the parent theme you use is being maintained you should have a nice solid base to work from.
In terms of what theme to use, there are loads of good 'uns. I am partial to my own theme, Elemental (
http://prothemedesign.com/) but that costs money so if you don't want to spend then I would go for Thematic (
http://themeshaper.com/thematic/), it's probably the best (most popular) free framework/ parent them around.