Would an Apple SC20 drive, with a 5.25" Seagate drive work?
Nope.
I don't have access to a Profile drive (I have never even seen one personally), and I doubt that seller has one either.
That's ok... it doesn't really matter. The only thing that the
III can boot from is the internal floppy disk drive.
Again, if I do get the III, would a connection between a Plus, and a III be possible to transfer files, or am I going over my head?
I don't know of any software that is amenable to data transfer between the
III and the old Mac crowd. Well, there is MacADT that does the usual .DSK transfers, now that I think of it.