A number of different bikes can work for commuting but if you want a bike that's good for commuting and long distance riding with a budget of $600, I'd look for a cross style bike. The drop bars are a big plus in long rides. The fat tires really help in commuting.

See post 4 above.

Something like this for $400, http://www.bikesdirect.com/products/...liberty_cx.htm

You'll have to build it yourself but you'll have money left over for lock, helmet, lights, and a rack.