Oh no not this old chestnut again.....
For full disclosure, my touring bike has BB7 road brakes. And yes, I do love them. However.... pretty much any good quality well setup common brake system will work (think about this, how long have people been touring?). I put discs* on my tourer as I liked the idea of not wearing the rims down, not heating rims on a descent, more predictability in the wet, modulation, etc. Truth be told though, v-brakes and cantis are fine brake systems (again, assuming good quality pads, well setup, well maintained, blah, blah, etc, etc) and each brake system has its advantages and disadvantages. FWIW, one of my favourite braking systems is a canti setup -the power and modulation is just outstanding! If I was building a touring bike again, I don't think I'd bother with discs -but I also certainly won't be taking them off my tourer anytime soon either.
If you are on a budget, I'd say save your money as disc specific hubs, possibly brake levers and the brakes themselves can easily be costly compared to more traditional rim brakes. And good rim brakes will work just fine -as many people can attest to.
*when this discussion comes up, I frequently hear the negatives of disc brakes that I find, quite frankly, to be -in my opinion -more grounded in the areas of trying to win an argument rather than reality. More complex? Yes. So are derailleurs and geared hubs -would you not use those? Reliability? I've had first hand experience of more trouble with rim brakes than discs.... Dished wheels less strong? I'm sure theoretically yes, but any good quality well built wheel -even dished for discs -should be fine. I don't see too many mtbers with disc wheels afraid of doing big jumps for fears of weakened wheels

Problem getting spares for disc brakes in Outer Mongolia? Yes, absolutely. But try to find a v-brake as well! (not to mention *most* people don't tour in really remote places.) Don't get me wrong, disc brakes do have their downsides (e.g. if you have a rear disc brake outside the seat stay you'll need a disc specific rack, there are less choices of brakes and levers for drop handlebars, less bang for the buck than their rim counterparts, less likelihood of finding spares in remote places, etc) but I do think some of the oft quoted negatives are just unrealistic and misleading in the big picture.