Yes in the end the choice of bike can come down to tyre sizes and availability. Trail bike size tyres are preferable because they are generally available anywhere. From experience 17 inch tyres as fitted to the R100GS are not available in Malawi.

Although you probably don't need a 21 inch front wheel for riding on 'roads' I should imagine it's really hard driving off tarmac with a fat front tyre. Anytime I tried to go off road on my Guzzi Le Mans always ended in disaster:(

take whatever
 

i rode the americas on a honda dommie, ny partner was on a klr 250 but one of our riding buddies had an old bmw boxer, 800cc i think but the road version. low handlebars, short travel suspension and about 25 years old. he rode all the roads we rode, some of the worse in bolivia and also patagonia and had just as much fun as us. he loved the bike, loved fixing it when it went wrong (which was often) and was in no way hindered by it. its all about how you feel about what you ride. if its good you can get most bikes most places. ride a bike that "floats your boat" so to speak and you will have a good time.
theres my two penneth anyhow
cheers
oz

whats right for you
 

I think bonnie man is on the right track here, its going to be a bike thats best for you, rather than trying to get one that sings and dances. I have a BMW R1150gs but I'm thinking of getting rid of it for a triumph & side car combo. The BMW is great but at the moment the Mrs wont get on it, so its not right for me.

know your bike
 

yep plenty good advice there again. of course you could stay on asphalt or decent dirt for a whole south am trip, but you would miss a lot IMO. and maybe do a lot of backtracking.

thing is the dirt roads can be disastrously rough, and may turn from decent to shocking without much warning or a logical reason (such as town, mine or whatever). also, i have found good maps of s. am. countries hard to come by and even newish ones are inaccurate. point is, be prepared. ground clearance is good, so are low tyre pressures. we are travelling two-up on an r11gs, and made space for a compressor.

anyhow was going to say, whatever you take, make sure you know it well in terms of maintenance and repairs, and take whatever tools you know how to use IMO. and a manual.

beg to differ with waterfox - just like all other makes, BMW sources parts from catalogues, so availability isn't that much of a drama in my experience. though he is right that there are plenty of honda signs around.

enjoy,
andy