I was thinking the same thing. That is a great bike and a great price. If you really want the Trek though, it is also a nice bike.
An earlier poster made a great point. Buy the most bike that you can because you will want a better bike later. There is another side to this, if you do not like biking, than you have wasted more money. I fell into the first category when I started. However, I am thankful that the LBS talked me down so that I could get what I wanted a few years later.
There is absolutely nothing wrong with Tiagra. You will not ride better, faster or longer with the 105 or Ultegra. If you stay within the low end of your budget you can always sell the bike and buy what you really want down the road. If you do not stick with it, you also limit your loss. Remember, when you first start you do not really know what you like. That takes time and things change. This sport is also intoxicating. There is always a newer bike and newer technology that we convince ourselves that we need or want. In the end, just don't get too drunk on the purchasing so that you go over budget. Have fun.