That would make my decision right there. If you get good vibes from the people you deal with, concentrate on that shop. Bikes from the major manufacturers are very good across the board, so customer service is the discriminator.

The CAAD10 is probably the best aluminum frame RACE bike you can buy. It's designed to provide the riding positions that racers and go-fast riders need. If you're less flexible, the CAAD8 (or the aluminum Synapse) would be a better choice. They're not slow - speed is totally rider-dependent - but most riders are more comfortable on longer rides with a slightly more upright position. If you gain flexibility as a result of riding, you can change the stem to get the riding position on the CAAD8 lower and it'll still look right; if you change the stem to get a higher position on the CAAD10, it'll just look funky. BTDT.

As for upgrading the drivetrain...get the best you can afford now. Upgrading is not cost-effective. Having said that, nine speed Tiagra is good stuff, and you won't be faster on a ten speed bike. The main advantage of ten speeds over nine is that your gearing is closer together. Lots of experienced riders have stayed with nine speed drivetrains.

Best of luck!