You'll want the most bombproof wheel ever. Nothing stops you dead in your tracks like a cracked rim. 48 hole tandem rim with heavy duty hubs. Extra spokes too.
Also, make sure the BB and Headset are newish (or newly rebuilt) high-quality parts. People tend to forget those, but a broken one will cause problems and is not worth carrying an extra part of because they are tool intensive to change.
I just finished a 3 month tour and here's what broke/needed to be replaced.
Rear wheel (2 times)
Tire (4 times -- based wear which is predicable, though you wanna have a spare just in case it tears)
Tubes (duh)
Chain (Didn't stretch, but wore out, granted I replaced it because it wasn't shifting well, not causde it broker)
Brake pads (1 pair -- Brake pads are not as easy to find as you think)
Frame (heh)
Spokes (Goes with wheels, but I went through 6)
Odometer mounts (Somehow these kept breaking on me and its a pain to navigate wthout one)
I might be forgetting something.
I would also bring a replacement brake cable and a cassette. And pedals (cheap, light resin ones)
You might not want to bring everything i broke, but its worth thinking about how you'd fix one.