The biggest biggest things I recommend to most people, in no particular order:

1. Make sure you at least get a carbon fork, for damping road vibration and harshness.
2. Get at least Tiagra level shifters for the ergonomics, Sora absolutely sucks.
3. Jump right into clipless pedals/shoes.
4. Make sure you get properly fitted.

105 isn't necessary. My Fuji Roubaix, Al frame with carbon fork and seat stays, Tiagra front, 105 rear, <20lb bike, $630 from Performance. Has served me great into my second season of riding now. No regrets. It's definitely not a factor in limiting my performance.