The key for reasonable airline baggage cost is to meet the Length + Width + Height <= 62" specification that qualifies as standard rather than oversized checked baggage. Don't see how you're going to get close to that with a non-folding mini velo frame and wheels. And quite a few of the rules for trains and other mass transit systems specify that for special, less restrictive treatment, a bicycle must have a 'true folding frame'. You might find a sympathetic conductor who looks the other way, but I feel much more comfortable knowing my bike satisfies the rules and I won't be subject to getting bumped off.
The Dahon already has a pretty wide-range gear set with a low gear of 32 gear-inches that should be adequate for quite a bit of hill climbing. It lacks a bit at the high end of the gear range for the downhills, but except for racing or keeping up with a fast group ride it's generally easy enough to just coast a bit more when descending. I'd try it for awhile as is before worrying about modifying it with a front derailleur - but it's nice to know that's an option.