presumably a lot of wiring and servos- uk cars used cable to control everything i think whereas j-spec cars had servos/motors to move air diverters etc.
Thats just the start- you have to mount and fit up the compressor, air con rad, that funny round silvery thing (condenser?) near the battery (i'm no a/c engineer huh?) run pipes through the bulkead, fit various relays, wire up compressor. If your donor car has full climate control theres also various temp/sunlight sensors around the car too.
I would like to think the loom is there. But i doubt it is

all in all, it sounds an awful job!