I am a 40mumblesomething year old, and have spent the better part of the past decade years living on various dung-heaps around the world, mostly desert environments. though I have 6 cycles in my stable I have one that is purpose built as the "deployment bike". I have ridden it in 8 Countries on 4 Continents and it has evolved into something ideally suited for nearly every eventuality, except for climbing really steep grades.
First consideration is always dependability, you want something that will not fail (break, wear out, or become fouled) when you are 1,000Km from the nearest bike shop, or stock of repair parts. or locally sourced repair parts will fit.
To that end a "single speed" is key. Forget the fancy derailleur and shifting mechanisms, they will get bent and broken and quickly foul with dust, dirt, mud and goat-dung. they add weight and lets face it most FOB's are pretty flat so you will spend 90% of your time in one gear ratio anyway.
The 29" wheel is the boss. It provides great flotation over the sandy, soft, muddy, gravel, etc terrain. and really it is nothing more than a voluminous 700c tire anyway. Pump it up for pavement riding, air down a bit for off road.
Mechanical disc brakes, last forever and have amazing stopping power, which is appreciated when an MRAP driver decides since he is bigger he is not going to yield your right of way. it also removed the wear from your rims, and the rubber pads are not going to disintegrate at high summer temperatures or on some long descent. I've tried them all and I would with the Avid BB-7. dead-reliable, easy to adjust, and inexpensive.
While a front shock is nice for comfort, but it adds lots of weight and fork seals will eventually leak, even a Marzocchi. The voume of the 29"er tire takes out a lot of the "harshness" of the rigid fork, so skip it, IMO.
There are many rides that fit the "rigid 29" single speed disc bike" out there, let your purse lead you to what you can afford.
This is what my bike looks like while bashing about the wilds of Afghanistan.
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