I don't think you are hurting your chances of getting parts. But, it all depends on where you go. Cummins parts are found throughout the world if you decide to go with the 4BT. Toyota diesels are great but you may be waiting for parts for a few days or so if you are in North America. Central and South America have a plethora of Toyota diesels. But, the same goes true with the Cummins. Parts are very plentiful in North America but you may have to wait a few days while in Central/South America.
I would not go with the OEM auxiliary tank as it is only 13 gallons. Not much bang for the buck. Go with the long range tank.
I am not sure about towing a trailer that far south. It is one less thing you have to worry about. Also, shipping your vehicle to South America will be more expensive as you just added an extra vehicle. Go with a roof rack and carry less spares. Also realize that the world is only getting smaller. If you do need parts that you can't get locally, Fed-Ex or UPS can deliver whatever you may need in just a matter of a few days. Expedition trailers are great but I don't think they are necessary or ideal for driving to South America.
I see that you have been doing your homework on the 4BT. I went with the Slee 4" on mine. I also have a 4BT in mine. You can get away with the OME heavies and some spacers but I prefer the Slee. Can't tell you why, just do. Seems like a cleaner install.
Also, PM sent about the 4BT.