You should check out the women specific models at BikesDirect: http://www.bikesdirect.com/primalstuff1.htm

At 5'1 even the smallest men's bike on the main page you listed will to too big for you.

Looks like there's two bikes in your price range: Windsor Willow and the Dawes Sheila. The Dawes Sheila looks like it would fit better on paper since you will be in between sizes on the Windsor Willow. But everyone's body proportions are different.

Spec wise, they are entry level as you may know. Looks like both bike have rear eyelets for a rack which is important if you want to use panniers to increase your carrying capacity and/or get weight off your shoulders and back.

I know southern california can be hilly. Do you need a wide range of gears??? If not, consider the Kona "Bike" http://www.konaworld.com/bike.cfm?content=bike At $449 it will come right under $500 with tax. Has eyelets for a rear rack. Has an internal 3-speed drivetrain so little to no maintenance required. I personally never seen one in person, but it is made of aluminum so I am guessing it looks more heavy than it really is. You would want the smallest model.