1) Schwalbe Marathon Mondial tires instead of Marathon Plus (+0$); I don't have experience with the Mondials, but can assure you that the M-Plus's will get you anywhere you need to go for a lot of years and with no flats (but aren't intended for off-road use). These tires standup very well even to heavy tandem touring with a total weight of 500 pounds.

2) V-brakes instead of canti (+20$)
Stick with Cantilevers and use KoolStops pads (take two spare sets). These will serve you much better over the years than the fad v-brakes. In a few more years the disk brakes will have reached maturity an then I will recommend a few of them, but for now you need solid reliable brakes that will work for you all tour long so it is Canti's with Koolstops. But don't start the tour if you haven't yet gained experience with tuning the (whatever type) brakes, replacing the pads, adjusting cables, and confirming that you have the tools needed in your kit.