ok that puts you on into the so called entry-level Road Bikes by good manufacturers, this means that you will get a good deal or good for the money transaction, as an example the Scott Speedster line, (but you need 800 usd) http://www.scott-sports.com/global/en/products/227904/scott-speedster-60-14-bike/ size for you would be XS49 or S52 (this is just one example, there are many other choices in the entry level market) or you can try your luck in Ebay and get a better bike for the same amount of money, like having more gears (9 speed cassette for example, or even 10), but since you will be riding mainly in the flats then having less gears wont hurt your riding too much anyway, a higher group for components, established brands like Shimano, Sram, Campagnolo, Truvativ and others, do have components for every price range, there are so many options it would be difficult to guide you there in the used market, there is obviously the good old bicycle shop option (referred as LBS) , where you can see things with your eyes and even test ride the bikes, you will also need to carry maintenance and buy accesories every now and then, so i would start by going to one of those, don't forget apparel, you will need cycling shoes, cycling shorts and a cycling jersey, and last but not least a cycling computer to record your speed and distance at least, there are many options to chose from, if you want an easier start buy normal platform pedals, that you could use with running shoes or common shoes, normal shorts and a t-shirt and later on as you get more enthusiastic you can start buying cycling apparel and shift your pedals into cycling clipless pedals, a helmet is a good idea at all times, and lights if you ride at night (those can be installed and uninstalled easily), lights are not for you to see the road but for cars to actually see you !