Funny how so many people that have not even looked at the bike in question's geometry chart want to chime in and tell OP that it's way too small.

The bike he's looking at while sized 50 has a 545mm effective top tube which makes it closer to a 54 or 55 in most frames. Bikesdirect claims it's closer to a 55-57 size frame. It has a sloping top tube and compact geo so short seat tube which is where they get the 50 but that is meaningless. Odds are 50 is the correct size for OP

As far as the bike goes: I've owned a BD bike and I felt like it was a good value. But might want to check out the used market. I saw a CAAD9 with 105 with a $750 asking price recently on my CL in the L.A. area. I've seen a lot of good deals in the $700 range here that would be better bang for the buck then a Tiagra bike. Miledge may vary depending on your local CL and used bike market in your area. Lots of good deals on Ebay recently for used bikes. I think lots of people upgrading for the summer and selling old bikes right now