All the photos plus descriptions are here:
https://imgur.com/gallery/Q90Jehx
Finally, all the ordered parts have arrived and I could assemble and build and test it today. These are two 4.5V 70s/80s train motors, one in the tender and one in the 7720 loc.
The tender also has a Power Functions receiver and a 4.8V NiMH battery pack. The charger is USB and the batteries can be charged via the plug, no need to remove anything.
I tried to push-pull 17 wagons with it, which was succesful (engines in the middle). Then I tried to pull all 21 wagons I own (engines in front), which failed because the magnets wouldn't hold despite added thin neodymium magnet discs.
I'll see about uploading a video if the quality is acceptable... Ah yes the roof of the tender wasn't finished yet in the video =>
And new attempt with all 21 wagons succeeded (23 wagons if you count the engines too): https://youtu.be/8dZ3aVw015E
Legotux