I would concentrate more on accessories than on the bike itself. For the length of your commute, pretty much any old bike will do, as long as you stay from department store bikes, which usually have inferior components and lack skilled assembly.

One universal law of commuting is that what people think is the ideal bike when they start commuting is traded in after a year or two of riding. I am on my third bike since I started commuting, but still use the same lights, fenders, rain gear, and panniers that I bought on day one. These accessories make a big difference in the enjoyment of your commute, in my case more so than my bike.

Get something comfortable and in good repair, and treat yourself to a new bike after a winter of commuting when you really know what you want. You will have saved enough money in gas to make a big dent into the bike you really want.