Yay for choosing South America! The vibe is totally different there than SE Asia, and I think much better for a first trip by an introspective person. (If you're a partier, SE Asia as young and irresponsibly as possible is practically a right-of-passage, and you will deal with many of these types in your travels there).
Where to start - let price be your guide! It doesn't matter all that much as far as pros and cons of one place versus another and what it's like to be there for the first time. Don't overthink it. I'm guessing Buenos Aires or Santiago will be cheapest to fly into. Maybe Lima.
