The Kalkhoff is still ideal Mike, for the obvious drive through gears reasons, and it's high power mode makes it better than the Lafree I used for years for towing in a hilly are.
Later I used an eZee Quando hub motor bike to which I'd added gears which was a bit faster at the job, though I probably wouldn't have changed if I'd had the Kalkhoff with the high power mode, the Lafree not having that.
The only two bikes I'd consider for this sort of towing are the Panasonic unit bikes or the more powerful eZee models like the Forte and Forza. The production Quando isn't suitable as it has no gears for the rider, and adding them is just short of impossible.
The web page below shows my sort of towing with Lafree and modified Quando:
Trailers
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