Chainrings are the front sprockets. The cassette is the set of rear sprockets, commonly called cogs. The chainrings on the Sirrus Sport are a good size for your application. You might wish you had a bigger cassette for a lower gear at some point, but what's there won't keep you from reaching your destination. It probably won't even make you get off and walk, but you might have a hard time pedaling up the steepest hills with a full load. The derailleur the bike comes with will let you go up to a 27T (27-tooth) big cog on the cassette. The difference from 25T to 27T is pretty minimal-- less than the difference between a 10% and an 11% grade. You could go up to a 30T, 32T, or 34T to get lower gears, but they would require changing the derailleur.
Many parts are easy to change out; IOW, you're not necessarily stuck with the parts that came on the bike. For more expensive bikes the dealer may make certain changes free of charge before you take delivery of the bike, like putting a bigger cassette and matching rear derailleur on if the retail value is about the same; but they will probably be reluctant to do it on a bike that's only $500-$600. Profit margins on bikes are quite narrow compared to the accessories; so don't think they should just do it for free because they're "making all this money."
A carbon fork will add less of its own resonances than a metal fork, and that results in a smoother-feeling ride. It does not absorb shock, but it sure makes the ride nicer. Carbon forks are quite a bit more expensive though. That upgrade definitely won't be free! A metal fork will do the job just fine-- it just won't be as nice to ride.
This field is rather information-intensive. Assuming the plan is to do this trip next summer, you'll have time to educate yourself on more details than you had any idea there were. Take time to ask questions and read up. For now just ride whatever bike you have-- lots-- and don't be in too big of a hurry to get the bike you'll take on the tour.
By the time you get all your luggage and camping gear and pay for your food and campsite fees, the bike probably won't be as big of a part of the overall cost as you thought, so there's no point in skimping if it would really reduce the pleasure of the trip.
You should probably take a shorter trip before the big one, just to evaluate the equipment and find out what you need to change, what you can leave behind, etc..
