Sleeping bag. I'm going to suggest a TWO BAG strategy. I would get one good quality UL down bag, rated to 35-40 degrees or so, something like a Western Mountaineering HighLite. Also a synthetic quilt, also in the 35-40 degree range (Enlightened Equipment Prodigy, maybe?) One or the other *or both* will get you comfortably through all of your climates and conditions.
With this set-up, you'll have a back-up in case one gets wet or damaged. Also, I'm going to guess that you may want to wash them at some point in your journey. Washing and drying a 0-degree down bag on the road will be a daunting exercise; a 40deg synthetic quilt will be much more manageable.
I think you're also going to want some sort of lightweight liner. One that's easily washable and quick-drying. Not really for warmth, but mostly to keep your bags cleaner.
Tent. I would personally choose a smallish 2-person tent, either double-wall or mostly so. I think the REI Quarter Dome is a great suggestion. If I had to choose one tent from among the ones I own to live in for a year, I would choose my MSR Hubba Hubba.
Stove and cookware. I would try to find a contact in SA who can talk to you about what kind of fuels can be found there. For instance, I communicated with 'Cafenet El Sol' in Costa Rica about hiking in Corcovado. You cannot fly with any kind of fuel, so whatever you choose, you're going to have to buy fuel at the destination.
I would start with a 1L lidded pot, small frypan (6-7") and a GSI kettle. You won't find freeze-dried foods on the road, so you'll need to be able to actually cook. You seem focused on sleeping bags but I think the food aspect is going to be the biggest challenge you have.

