If folding into a suit case for air travel is not a prime objective and you like the classic vintage look, then buy a Raleigh 20 and mod it to your needs. They are very robust and can handle heavy loads and long tours with relative ease, unlike some cheap and flexy modern folding bikes. Use quality components and you wont have the problems some folding bike brands experience because of questionable comps.
If the thought of building your own bike is not your thing, then Bike Friday's New World Tourist is an excellent choice for touring...and it will break-down into a suitcase.
I would be hesitant on any serious touring with a Dahon, though their owners would disagree, IMHO, they are not strong enough.
