Buying a less expensive bike with the intent to upgrade stuff later isn't usually the best way to go. You will end up spending more in the long run for less of a bike than you could buy "off the shelf".
If you don't want to purchase used, I'd recommend either buying an entry-level road bike (I bought a Schwinn Le Tour GS a little over a year ago for $500 @ Performance when I was about your weight) and plan on buying a better bike when you are ready for it. You can then sell the entry-level bike or keep it as a second bike (when I bought my new road bike, I kept the Schwinn for commuting to work). Or bite the bullet and buy your second bike first.