I've toured on a Dahon which was ok but the components just wore out, including the folding mechanism hinges. I own a Brompton but wouldn't use it for touring unless it was a trip with more public transport than cycling.
My recommendation would be a Bike Friday with 20" wheels; I've toured in a number of countries around the world on my Pocket Llama, much off road or on pretty rough, rocky surfaces, including mountain ranges with big climbs & descents, with full camping gear. It's been fantastic, & handles absolutely as well as my tourers (better than some of them
) The only difficulty has been keeping the weight down for flying when taking it as normal hold luggage & packed into its hard case. The bike itself comes in just under the weight allowance of 23kg, but tools & a sturdy lock often tip it over the limit. However, you could obviate this by checking it in as a cycle & paying the fee. I've also toured with it in Asia with a soft bag, carrying the necessary padding, & using trains, buses & planes - this has been fine too although, as with all bikes, packing is important.
Mr AMC has a New World Tourist which he's used more on tarmac/gravel, but which has been a comfortable and well-handling load carrier too; I'm sure it would do pretty much anything you need it to. (The main advantage of the Pocket Llama is the clearance for fat tyres but you may not need this if you're not on very rough surfaces.)
The only disadvantage I can think of with 20" wheels is in countries with wet seasons and very deep mud, which would be best avoided. Agree with Simonhill though that carrying a folding bike separately to all the panniers etc can be hard. I've only done it on Eurostar as it required the bike to be bagged, & we had the bikes in specially made very lightweight bags over one shoulder & the panniers/bar bag etc in one of those big checked laundry bags. This was harder for me as I'm shorter than Mr AMC who found it less difficult lugging them about. It is easier if you can just wheel the bike around with the panniers on using lifts etc., so might be worth checking out the train company regs on this.
Good luck - I do recommend touring on a folder as it opens up a lot of transport options which can be very useful.