There is a lot to do before going but as planning is part of who I am, I enjoy it immensely!
One of the biggest challenge is to face the French Insurance system between: health insurance, travel insurance, personal insurance, damage insurance, social security, mutual... If you not crazy before starting this process...you'll be after! I'm leaving in 12 days and still don't have social insurance...that's going to be tricky but not impossible!
FYI: If you are French, not a student anymore but still not working, the LMDE will not cover you. It's kind a gap in the insurance coverage. You have no choice but to ask to be covered by the CFE (Caisse des Français à l'Etranger). But be prepared to wait some days/weeks before having an answer.
Another important part of planning the trip is to buy everything you need : shopping, shopping! Shopping!!! :D
Of course the biggest cost is the plane ticket! The best is to check on Lufthansa (www.lufthansa.com) or E-dreams (www.edreams.co.uk) or at least it's where I've found the cheapest rates. The second one is great as it offers thousand of companies and you just have to chose which one you want depending on the price, the time, the numbers of stops...
In terms of vaccine you will need : Malaria, Yellow fever, Hepatitis A, Hepatitis B, Tetanus-Polio-Diphtheria, Typhoid fever and Rabies.
A good aspect of going in trekking in South America after my job is that I will have to carry everything I take with me... Do you know a better way not to take unnecessary things with you? ^^
Of course, for that you also need a good hiking bag because trying to climb a volcan dragging your suitcase is...well... insane even for me!
I also trade my large wardrobe of dresses and high heels (LOL) against good walking shoes and canvas trousers... try to dry jeans during the rainy season... ;-)
