Thanks for your reply, bluefish. Let me start by saying that we don't actually plan to do a lot of miles; this is intended as sort of a hybrid of bike tour and just standard vacation. So maybe 30 miles per day, 40 on a "big day". Leaving a lot of time to stop along the way and to explore the destination town each afternoon. I agree with you that if I were doing high daily mileage I wouldn't be keen on compromising too much on the bike choice.

FWIW, there's a surprisingly vigorous community of folding bike enthusiasts. I guess that's true for just about anything these days, however, given the power of the internet to connect folks with similar passions.

People definitely do long distance rides on folding bikes. Wheel size is just one of the factors that make this work; having a gearing system tuned to the wheel size is of course a good idea (!). But overall good components, proper frame size, a good seat, lots of stuff matters here. A smaller wheel makes a bike "more twitchy" according to detractors, and "more responsive" to supporters. We'll opt for "lighter, cheaper, simpler" and so not get any sort of suspension, but that's an option for folding bikes too that can make a difference, depending on typical trail/road surface.

I will say that when we tried out a Brompton bike with 16" wheels we were less comfortable than on a low-end bike with 20" wheels. Brompton makes a somewhat expensive and very popular folding bike that is tuned to the urban environ, folds VERY small and is quite light, but only comes with 16" wheels. So at some point I think that wheel size does get to be more of a factor (this with a very very limited opportunity to compare). But I was surprised at how comfortable we were on a 20" folder.

Note some of the special benefits for bike touring also. Rather than wait for a special bike car on a train that has one, we can get on any train at any time. Put a lightweight soft cover over a folded bike and it's just standard luggage. Ditto bus or taxi. Also could help with bike storage in hotels --- don't need a "bike friendly" establishment, just carry it to the room. Which also means it's more secure and out of the weather.

We did think about buying locally and then selling at the end of the trip. On the buying end, my very limited experience suggests that you can't just go into a bike shop and walk out a half hour later with a bike that's all tuned and ready to go. Perhaps if fortunate, and willing to compromise on what you get. It's the selling end, however, that really concerns me. We would want the bikes right up to the end of the trip, and then we have to get on a plane and go. I don't see how that could work. We're hoping to do this sort of vacation more than once, so that also suggests better economy in owning bikes we can take with us.

Breaking down completely is perhaps an option, but even then I think that you're paying oversize charges to take them on the plane or ship them ahead. If that's the case, might just as well do only the more minimal disassembly required to put in a standard cardboard bike shipping carton. But then you have to typically book directly with the airline and make sure they have space for your bike, and then pay up, and it's typically not cheap, perhaps $150 per bike. Here's a fairly recent reference: http://www.pinkbike.com/news/5-tips-for-flying-with-bicycles-2015.html

I realize that you have experience to the contrary, but how typical is that? Today, btw, when airlines are always finding new ways to generate revenue?
A full sized bike has a 26" diameter wheel. That means that at the minimum, a box that carries it is more than 26" on a side. The rules for most airlines today is that it's oversized if the sum of length, width, and depth are greater than 62". With a 26" wheel, you're already over 52" before you get to the depth. The kicker is that the frame size on a standard bike is way outside of that --- about 40" long on my bike. No way can a standard bike be cut down to fit in a box where the three dimensions sum to less than 62" short of using a hacksaw in the process.

Rats, I don't mean to sound argumentative here, just laying out my thought process --- please take the above as such (!). If I'm missing something in my analysis, I really would appreciate knowing that, so thanks for the interaction (just laying out my thinking like this helps, so btw also apologies for the long response).
_________________________
Brian Lewis
http://postholer.com/brianle