I would give some serious thought to buying used at that budget. If you go full roadie, almost immediately you'll probably want to upgrade to spd pedals and shoes, which will be an extra $150-200 You're looking at another $200 or so to outfit yourself with a few modestly priced jerseys and shorts. Then you may find want to upgrade the saddle, and maybe your helmet, bike computer, etc. all of these are small comforts that make road biking enjoyable.

My point is that you $500 on the used market will get you a 5 yr old bike with 105/tiagra, aluminum frame, and carbon fork. Spend the rest on gear and upgrading your bike to suit your needs. If you absolutely love it, then save your pennies and upgrade next season if you find you've outgrown the bike.