What Gios model do you have? Is it the Pure Drop
Why are you swapping the fork and front brake to what you ordered? If you have the Pure Drop, the fork that's on it will ride better than a carbon fork.
As stated, if you're parched, you're probably already dehydrated before even starting. I make sure to down one 22oz bottle of water per hour, on the road, while sipping on a Gatorade at the beginning of each long ride. I, also, drink about a gallon of water, everyday. Yes. I urinate a lot.
You'll have to try seats, before buying. Most shops will allow you to do a test ride, before committing. Currently, I'm on a Brooks B17 Imperial (in addition to the cutout, the crown is different than a B17). I've found that I can pretty much tell if a seat is going to workout, within a mile. If I have pressure points, I know it won't work.
I think you'll find that the cost of upgrading your drivetrain to Shimano 105 is going to be fairly expensive, relative to what your entire bike costs. If you're just wanting a double chainring, then, look at installing a compact crankset and leave the rest. Compact cranksets are available in 50/34T or 48/34T. Your LBS can help you select and install the right components.