I 2nd bikeguru's suggestion in regard to using 26" rather than 700c. I'm using my Waterford Adventure bike for my tour-it's 700c. I left Paris mid April, I'm currently in Kashgar.
I've had a heck of a time with tires: My 3 Schwalbe Marathon XR's all failed very prematurely (bead separated from sidewall on all 3). I've looked for suitable replacements since Ukraine, and am currently running a Czech & a Chinese tire. Had I been running 26", replacement tires would have been no problem. Furthermore, suitable 26" wheels can be found in most major cities (Ukraine, Russia, Kazhak, Kyrgyz-and likely China as well).
Schwalbe is currently sending me some warranty replacement tires here. Excellent customer service (outstanding actually!). They must have had a bad production lot of there 700c tires or something.
-on the topic of a frame choice-
I'd keep an eye on e-bay for a early/mid 1990's steel hardtail MTB. They should be available for a song. Most should have eyelets etc, just upgrade the drive components if need be. Also you can still get them with chromo forks, rather than suspension.