I'm writing up the proposals for the "clients".
What I do know up to date.
Renting 2 comfy "cruiser" e-tadpools (my first toughts, Azub TiFly or HP Velotechnic Scorpion FS) for a month is not possible in Belgium.
Making sure it's a carefree vacation trip without a support technician on hand is . Even a brandnew commercial bike/trike will develop some creaks/hickups/flats on a trip like that, especialy with "drivers" that ain't used to the machines.
Transition from upwrong bike to recumbent does hurt. You'll start using other muscles and/or in another way.
My toughts.
If the clients are determined to do this trip, they will need training, 100km/day for 2 weeks is not something you do on a whim. Starting a few months in advance is not a luxury, unless masochisme is in play.
Commercial E-tadpools configured for long range cruising ain't cheap, and take a while to be delivered. (think about 6 months+ delivery time, and even then, limited in power and range)
My recommendations for now.(with some idea's from others, thanks guys)
Buy second hand , and upgrade with the right parts.
Go full Atomic Zombie. Building 2 Streetfoxes won't take much more time than 1. And that time will be about the same for a Streetfox'd Viking(appropriate tough, for the trip).
Start way in advance (preferably next week), so training and working out kinks won't interfere with the trip.
Have a support team at hand ( even the best road assistance takes hours of lost time)