I second the idea of buying prebuilt unless it's the fun of the project you are after. You will spend more money doing it.
Googling "Singlespeed commuter city bike" brings up:
http://urbanvelo.org/singlespeed-com...ikes-for-2009/
http://urbanvelo.org/singlespeed-com...ikes-for-2010/
BikesDirect has the Essex and you can get it with color matched fenders and rear rack, too.
http://www.bikesdirect.com/products/windsor/essex.htm
Schwinn has the Coffee
http://www.roadbikeoutlet.com/2011-s...mpaign=product
here's somebody that built his own commuter bike:
http://www.outsideonline.com/outdoor...uter-Bike.html
The Trek Soho
http://bicycling.about.com/od/bikeeq.../Trek_Soho.htm
I'd be hitting yard sales, estate sales, Goodwill stores, etc. to find a 70's-80's road frame like a Raleigh Grand Prix, Record, Sprite or a Schwinn Varsity, Collegiate, Super Sport, etc. Or a Miyata, Univega, Bridgestone, Fuji, Panasonic; a lot of the 80's Japanese steel bikes were well made and still undervalued. Many of these will have brazeons for fenders, racks and can take wider tires.
You could pick up a complete used bike for $100-200 then use it and most of the parts to make your SS commuter. If the rear wheel has a freewheel hub, you just remove the old cogset and put on a new singlespeed cog. A lot of the 70's bikes had 52-42 chainrings so you could keep that crankset and use a Surly Dingle to give you more choices for gearing.
I took this:
and made this:
You can see I have a double crank with a Surly Dingle with a freewheel on the other side of the flip-flop hub. The long dropouts on the Sprite frame means this bike has six different gear options; four fixed and two free.