I have aluminium bikes and they can be harsh but the latest one has C.F. forks and seatpost. I can feel the harshness of the ride with the noise it makes but it does not get through to me so that could be one of your doubts done with. Cannondales make a good bike- They don't know how to make any other. They ride well- last well and a 10 year old "Dale" still gets appreciation. 105 gruppo makes sense- good without being extravagant, and my only concern is as to whether it has a triple or double crankset fitted to it. If you ride a triple at present and use it- then you probably will want to stay with a triple.

I'd say think about it overnight- Check that it is completely to your satisfaction by a test ride- get a few anciliaries thrown in like Spare tube- Pump and anything else you can cadge- and go for it.