I'm still in the extremely early stages of researching this fantasy. I am thinking car, and I'm also thinking of buying it in Mexico, to not only save some money on the price, but to get something that is common currency from Mexico to Argentina (maybe a Toyota 4wd?) I'd drive my own car, but I kinda think that Washington DC license plates would stand out like a sore thumb and become a potentially very real security issue on large parts of the route. Mexican tags, even if foreign, wouldn't be nearly as strange in Colombia, Ecuador, Peru, Chile and Argentina, and would be far less attention-getting in Honduras, Nicaragua, and El Salvador.