The 165 might be easier to work with depending on your plans as the 165 supports SCSI disk mode and the 140 does not.
And who could say 'no' to the passive-matrix-yet-still-quite-nice 4-bit grayscale LCD in the 165?
Sorry, I forgot to check back here. :)
50-pin 2.5" SCSI is 50-pin 2.5" SCSI so you should be able to swap drives. I've never tried, but you should be able to. Someone at 68KMLA can give you a more definitive answer.
Funny how your grayscale screen almost looks like a colour one when photographed—purple and black and white!
120MB is a reasonable stock drive for a machine of that age. Only the original 100/140/170 ca. 1991 series topped out at 80MB.
I have $20 here that says the author of Mactracker doesn't have a 165 against which he can verify that information. :-) I don't have real hardware to check either but I do know that:
everything through the PowerBook 100-5xx series used SCSI (except for the 190)
2.5" IDE disks are 44-pin, not 40
there's only one 50-pin 2.5" SCSI connector used in PowerBooks
Open up your machines and check for yourself. :)