By saying "eighteen" gears or more, posters have meant to say be sure to get a triple chainring. This is absolutely a necessity for touring. My old bike shop boss gave me a bad time for ordering a triple crankset on my road tandem. Even without the added load of bags & panniers and/or trailer, we've used the small chainring lots!
I agree with another poster that a Bob trailer is a great way to go. If you have to keep the budget down, a trailer will lessen the weight that will burden the components when loaded. However, be sure to include either a drag brake or disc brakes in the equation. Unloaded tandems overhead rims very quickly on descents - touring much more so.
Now the flying question..... I once received a call from a frantic woman when I worked at a bike shop. She was flying to France in a few days and couldn't find a box anywhere. Now this may have changed, but you definitely want to be prepared for travel with a box obtained well beforehand. And even then, boxing a tandem is not a simple task.
If you're travelling by plane frequently with it and you can take out a second mortgage on the house, consider a tandem with S&S couplers. This allows you to carry the tandem apart in suitcase-sized cases. Best protection without the added cost and hassle of a tandem box. But be sure you're sitting down when you see the prices of S&S-equipped tandems PLUS the prices of the cases!
I hope this helps.