I have the CAAD9. It's awesome. Cannondale know how to make good aluminium frames. A couple of CAAD8s ridden as second bikes by ERCers I know and they are pretty happy with them. Certainly good for sportives. I've had no problems with the CAAD9 frame and it's a huge leap up from the cheap Trek 1000 frame on my old road bike. I've had no problem using it long distances, with a decent saddle and carbon fork and seatpost and I certainly aprpeciate its lack of weight over the steel touring frame when it comes to going up the hulls. For that sort of money I think you'll get a better aluminium frame (your in the middle of that market) over a steel frame (you're at the bottom of that market).
Some people dislike Sora STIs as you need to use the thumb to release the cable tension (i.e. shift down at the front or up on the rear) and if you don't have long / flexible thumbs find it hard to shift on the drops. I have Sora on the Trek and I've found it very reliable and workmanlike, but agree the STIs aren't the most convenient layout. The mechs are fine and Shimano do a non-range STI with the more common setup of a 2-part, finger-operated brake lever.
Singlespeeds are great in my opinion but wouldn't want to do a sportive on one - if you got a headwind or a climb when you're tired out you'd be a bit scunnered - also you don't get to pick up tailwind advantage if you can't get into a gear that allows you to pedal at a comfortable rate.