Many foldies uses road bike rear derailleurs. I am just suprised that someone said that it might hit out of alignment   All RDs will go out of alignment sooner or later. The chain might elongate, the hanger might bend, the cables have stretched, so many reasons. That's why one should learn to adjust the RDs. Or send to a bike shop to adjust. Probably Brompton owners said that statement, because Bromptons use Sturmey Archer internal hub gear (which off course don't run out of alignment    )which I find bothersome as it uses no quick release. You have to carry a 15mm spanner on your rides as you need it if there's a puncture. 

If chromoly is needed , which is more comfortable to ride anyway, just go for the Dahon Speed P8. First of all, the frame is really proven.  Second, it is within your price range, Third , it is really comfortable to ride. I think you should be able to get it for less than 2K from Kimbell Jaya. Add a 39 chainring, an FD and adapter plus a Microshift 8 speed shifter pair. another cable and housing, kautim! Same, same , 16 speed also. Lagi syiok, get the SRAM X5 shifters set, 10 speed cassette and make it 20 speed. The new Speeds are now shodded with SRAM X7 RDs.

For touring, it is good to have extra chainrings for some hilly rides.

This comment really put you in a dilemma    

I have a Dahon Vitesse for sale, with Shimano 105 double compact cranks and 10speed rear derailleur.  Schwalbe Durano tyres.  Under 10kgs. Hardly used since built in 2014.  Can supply parts list for this bike.
Asking RM1800.  Available end of Feb.

As for the RD, unless you mis-treat the bike, or transported the bike lying on the RD, I can't see how it can go out of alignment.  My bikes never have done it.  Derailleurs are much easier to maintain if you ask me, and I have bikes with internal hubs, like Sturmey Archer and Shimano Alfine.  Internal hubs have wider spread gears, hence can be either too high or too low sometimes, which is a nuisance.  And they are a little heavier also.

Touring speed is usually quite low, say 18-20km/h, so 8 speed will get you there if not geared too low, and if you want to be able to climb hills seated, then a second smaller chain ring would be required.  An internal hub will take you up hills so long as they are no too steep, ie. 8 or 11 speed Alfine. Even my Brompton 6 speed (with 3 speed hub) can go up pretty steep hills easily, but I won't want it to be a long continuous climb!!

Only problem with the 24" is the lack of tyre choices, and of course the larger fold. But as you say, it should feel more stable.

How much?