I'm not getting a 26in bike. They ride horribly, look silly and anything that'll crack a 36h 26in rim will 99% break a heavy 36h 700c rim. I weigh 65kg (140lb) and shouldn't need more than 20-25kg of stuff. 80-90kg is a lot.

Half of the trip will be in North America, where spares are common.

Half of the trip will be in South America, where, as far as people have stated, 700c tyres are available, if only 'racing' tyres. I'd be more than ok to use one in a pinch. To get to that stage i'd have to (1) destroy 2 schwalbe marathon plus tyres beyond what they can be booted. (2) destroy 1 or 2 spare tyres.

So i'd have to taco or crack a rim. I don't weigh much (65kg - 140lb), have never broken an alloy rim (even sub 400g ones) and they're is 35c worth of tyre protecting any hits, regardless. They'll be 2200G+ wheels (ie, very strong). If it does so happen that I end up out of luck in a place where I can't find a spare 700c wheel, I could get to a major city, stay in a hotel and mail order a new wheel. Wiggle, CRC and starbike get to everywhere that I have ordered from within a week. 26in tubes fit in 700c wheels, so that's not a problem either.

So yeah, not willing to ride a frankenbike for 6-9-12 months just because I might be a bit further, and a few hundred dollars away from parts than I otherwise would be.