Awesome trip and idea. Exposure to foreign cultures, being outside your comfort zone and challenging yourself will make you a better person. Don't listen to old codger fear. Having done the trip south last year it is so totally worth it. However there is a plethora of information allready available. If you're on here asking for basics such as paperwork requirements, I'd question your resourcefulness required to do something like this. TBH if I was 18 with 7k, screw the expensive US, fly to south america and bounce around hostels and backpack, remove the drama of the bike. Pick your journey as it comes based on the people you meet along the way. Use local buses. Buy or rent a local bike if you must. Party, see the world and have the flexibility to bounce around, then easily come home when you're broke.