I think alot depends of variables; how heavy are you,how much gear to you plan to take, what type of terrain, how long you plan on "travelling", etc. etc.
Don Heath knows what he is talking about.
To me, if a person in not too heavy, and is not carrying alot of extra weight, a small 4 stroke, single cylinder, kick start is ideal. Even if you have to gear it down a bit ( bigger rear sproket). An "enduro" I think is best ( street legal, hence, a decent headlight, even if you eliminate the tail and turn signals for stealthy movement ). Perhaps "cone off" headlight, for a narrow beam, with little side illumination.
Good rack, perhaps ammo cans mounted on rear sides, etc.
Great milage, dependable, easy to repair. Perhaps add an additional in-line fuel filter. Something I'd like, but is not readilly available, is an enclosed drive chain. I was down in Baja some years ago, and a Mexican Honda ( 125?) had this feature, keeps the chain clean, less maintainance, and maybe a bit less noise. Speaking of noise, that is another 4 stroke advantage. IN FACT, if I was prepping a vehcle for what you describe, I would very seriously look at adding an additional muffler to futher quiet it down, like what is available for ATV quads. Another big advantage to a small bike is light weight, easier to manhandle if you have to, and for that matter, if you need to fjord water, easier to float using improvised matterial. So what if top spead is 25-35mph, beats walking or pedeling. A person can find such a bike pretty cheap, as many are bought for youths who quickly outgrow them.
Mark