The computer is behind the glove box. The wiring harness is the one for the engine, but I don't remember if you NEED it or not. I had one so I used it.
Yes they can be pulled from the donor car.
The clutch start switch will have to take some wiring. It's pretty much just a switch under the clutch that when you push the clutch to the floor, trips the switch and lets the computer know it's ok to start the engine. It's just a safety thing to make sure you don't start in gear, so it doesn't affect anything. Just don't start it in gear...haha.
Anyway. To wire it you'll need to find the black and white striped wire that looks like maybe a 10 ga wire that runs right up to the ignition module. Cut it. Splice in a wire long enough to go to the engine bay fuse box. Make sure you splice it on the end that comes from the harness, not from the ignition module. Then you can just tuck the wire under the starter relay connector.
Mine looks pretty slick, you can only see the wire if you're really looking for it. I took the entire fuse box apart to wire it, and I don't recommend doing that hahaha.
Anyway. If you have other questions let me know, as I just finished the swap two months ago.
Check out my cardomain page for some pictures too!