A good enough rider can use just about anything. So, here's mine, keep in mind that I'm 17, so things like "comfort" aren't really a priority. A KTM 150xc, that's right, high strung 2 stroke single track killer. But hear me out. It's about as simple as you can get on a dirt bike. Put on some over-sized radiators, an oil cooler, armor it up, toss on a massive gas tank and you're good to go on the crappiest roads you can find.
The radiators and oil cooler should bring out the maintenance intervals a bit. Hell, given the size of the engine, I'd bring a spare top end kit or two. Look, even if a KLR650, which is as about as simple and reliable as you can get, has a "piston event", which is unlikely but still possible. Your little adventure has hit a pretty big bump. But rebuilding a 150cc two stroke? You could take it to a shop in town and have it rebuilt by the next day if you have the parts. Granted, a klr650 probably wouldn't grenade in the first place. But think, if you used a 150xc, you have a sub 250lbs adventurer. But you won't using heated grips or any other neat do-hickies. I think that the greatest benefit is not that it's fast(although it is a very capable single tracker) or cheap(Yeah, KTM stuff is pricey), but that it's easy to maintain, and to repair in the event of engine damage.
Oh yeah, that picture is Luis Belaustegui, attempted the 2011 Paris Dakar on a 150xc . But had to wash out due to being 12 minutes late to stage, due to stopping to assist many riders along the way. He finished on the assistance route. Also made a point that the bike had no mechanical failures during the race.