Following up on my pondering, I went with an enclosure for the 120 Gb drive, so the total for a portable backup store comes out to about US$80 (the enclosure was $19 but s&h was $15: feh).
Now the next gap to fill is some memory for the aging iMac SE. It needs 8ns PC100 SDRAM, as much as will fit. It has two slots, currently occupied by a 64 Mb stick to augment the 128 Mb it shipped with. So it looks like a 512 Mb stick is the way to go if I want to breathe a little more life into it. Perhaps two if it can be done. 640 Mb (512 + 128) will be a lot better than the current 192 Mb.
[composed and posted with ecto]