tinlizzy: the wiring on them often needs replacement and is relatively easy. There are heat insulated replacements especially for applicances like this too. I have come a cross a few mentions of people keeping the foldable iron in the Featherweight case. I think there are two types of cases for the Featherweight, minor diffferences, with or wtih out the tray. Not sure if it fits both of them, but they can just bout close the lid on case.
I don't know about ultimate accessory, it depens upon how attractive the old iron is to someone. There are lots of nice accessories that work for the low shank 221, the elusive Penguine (hardly obtainable), the Swiss zigzagger, some like the new Westalee ruler foot. I don't think there were a lot of accessories made especially for the Featherweights, but compatible with most of the low shank feet. I think the 121094 darning foot came with in the 221 accessories box, but also standard with the 201 accessories. I guess it depends upon what you are most likely to use.