Sorry Phil, that's 25 years I ride motorcycles (in the US, Africa, Europe, South America) and your project is really an utopy.
For less than 4500 pesos you won't find any motorcycle really reliable. You can purchase for that price a brand new motomel CG 125, the engine is quite reliable (but that's about it) and you are going to suffer on it. You need at least a 200/250cc to travel around South America.
Since you have no experience about motorcycle : the first time you'll need to cross sand or mud, you are going to crash, that's for sure (why ? because once you start losing stability your first reflex is to pull the brakes = on a motorcycle it's the opposite, you need to accelerate in order to regain equilibrium). You might end up in the middle of nowhere with a broken wrist (broke my wrist twice in 25 years, two times my legs, both my knees, my back).
Furthermore if you buy a motorcycle in Argentina, you can't take it out of the country.
The best you could do would be to drive from the US down to Argentina = after 2000 miles you'll be able to see if you can do it and head backwards if necessary.
It's a very nice project though ! But for 4500 pesos = no way
