Ya get on RS25.
For an OBD2 donor you will probably want an OBD2 car, so an 98-01 L or RS.
Airbag stuff is separate, and will only be affected if you swap the dash or steering wheel.
Basically the STI harness has remaining GC accessories spliced on. Cluster, fuse and relay boxes, pedals (way easier to use a manual GC pedal setup) and other accessories. STI harness is used for ECU and engine. Your STI DBW throttle pedal bolts right in with an OEM adapter that costs $12z
I personally would not bother with the dash as they never line up perfectly, but to all their own.
If you use the STI steering rack you need a steering knuckle adapter. Don't bother trying to swap the STI steering column.
Take the complete sub frames from your STI and swap em over. They will bolt right up.
You will need 90-91 legacy rear Tophats to make your STI rear springs bolt into the GC chassis. I have the part number somewhere.
There are a couple threads on radiators, but the STI radiator will fit. Read up on that.
That's pretty much it. PM me if you want any details