I doubt you can add disc brakes later; you need special tabs on the frame to accomodate discs. So if the idea interests you, I'd start with that up front.

The main advantage is that discs will work better in bad weather and require a little bit less maintenance overall. That's about it.

Offhand I don't think there is a huge need for fear over carbon forks, but I would agree that if you plan to tour with front racks, you're probably a little better off with a metal fork.