Tiagra is fine for most people, but I love the 105s on my bike. If you are going with disc brakes I would definitely recommend the BB7, even if you have to pay for an upgrade when you buy it (work with your LBS about it).
Cyclocross season will be over in a few months (Feb) so if you can wait I'm sure you can find some killer deals on either used gear or old stock that didn't sell soon. If you decide to go that route, and I would, I would wait.