I have lived and ridden in SA for 30 years and I agree all the others! I love the XRR too, but it is too high-tech for the riding you will be doing and you will have to adapt every accessory!
The KLR has a bigger following with expats riding throughout SA and they are fanatics to the point that they will even share parts if you are in a pinch. But I personally would go with an XRL (in fact I spent 36 days on one this summer riding every dirt road all over Peru) and as it has not changed AT ALL since 1996, they are easy to come by and parts are quite inexpensive in the USA. I like to off-road and the KLR is a head-heavy beast good mostly for straight line riding. Also, many parts supplied all over SA for other Hondas also fit the XRL's. Tires, chains and sprockets, levers, brake pads, etc will be quite easy to find. The only advantage to the KLR (IMHO) is the more comfortable seat, but then I mostly ride hard standing up.
My recommendation is to carry virtually NOTHING! Get a bigger plastic tank, a rear rack, and a very comfortable (w/ internal frame) slim back pack. Besides my riding gear I only take: 2 changes of clothes, a pair of CROCS, 2 lbs of tools, an extra tube for each end, water bottle, camera, and a pair of tie-downs.
If you break down, you just flag down a truck and tie down the bike. There will be everything you need at the next town. Don't camp and don't cook! It is safe and people are friendly. There are small good hostels w/ hot water and internet EVERYWHERE, usually for under $10/night including breakfast. You can even find their websites online.
400km pavement / 250km gravel are good days. When you push more than that you have problems and miss all the good stuff. Don't plan riding for days back to back to back. Your bike needs days off too. Two "on", one "off" is the heaviest schedule I would keep. I've done the back to back to back, through rain and snow, etc. You finish the trip and feel like you never even saw anything!
I've riden alot in the Andes, up to 17,000 ft and have some recommendations.

PM me here or email me. I can also recommend (as fresh as 2010) many, many roads in Peru, Bolivia, and Brazil. DON'T DO THE !@#$%^&* PANAMERICANA! You see NOTHING!

Leave that to the 2-wheel trucks! GO INLAND!
Where in the US do you plan to start from? I can hook you up with great Honda dealerships (with discounts) in Virginia and North Carolina who are personal friends and love to see guys do these trips.
Where are you coming from in Ireland? I know every American likes to claim Irish heritage....

but as a Shannon I DO have some ties!
Blessings,