I don't recommend used calipers. They will VERY likely be crap. Imagine this...caliper sitting for years without use.
Rebuilt ones are only about $80 each.
Also, NO...only MKIII calipers fit MKIII's.
They even sell loaded ones (come with new pads).
AND...read this thread, especially the posts towards the end...about the seized bolts and crap.
http://www.toyota-supra.info/forums/...ead.php?t=3654