I have one of those Franklin Mint Junkers F-13 floatplanes. It's spectacular, and I think worth the price. That was actually the world's first commercial airliner, although it could only carry 4 pasengers in the small cabine. A late 1920's airplane with much bigger capacity is the famous Ford Tri-motor. Also, I believe there were later larger Junkers leading up to the Ju-52 of WW2 fame.
I'd suggest that plane selection should depend on the size of the group of figures to be transported. If you want a couple of figures, then there's plenty of 2 seat airplanes. For a group of six figures, that F-13 is perfect. For a group of a dozen figures, a Ford Tri-motor would be a good choice.
Given the scale, a scratchbuild would be a real possibility, especially since these 1920's aircraft were somewhat boxy. The plane could be stated to be an experimental model, so it wouldn't have to be modelled on a historical plane.