I'm planning to do some on my newly acquired Twenty. The upgrades I have planned and in process are:

1) Gearing. 46 tooth front (maybe the original crank) and the "double sprocket" trick in the back with a 16 paired with a 22 for a bail-out gear for hills, both on the SA AW 3-speed hub.

2) Better front brakes. I cut the head-tube down and installed a 20" MTB fork with a threaded headset. Then canti-brakes on an aluminum rim. I have a quill stem steerer extender and a tall quill stem for it to get the height to a manageable level.

3) Racks. I have my doubts about the Plescher original rear, but I haven't decided yet about its replacement. The front can be a canti-stud mounted MTB rack, cut to 20".

The rims and wheels and SA hub are plenty strong, IMO.