Looks like a good start.
What are you using for your encoder? Hacked controllers or planning to drop in several of the new KADEs once they released?
Have you made a list of admin button functions? (start, back, exit, etc.)
Also, I assume on the rainbow 16-pack of wires that you are getting several of them:
2 joysticks - 8 wires
12 player buttons - 12 wires
other buttons - ?
To see if you've forgotten anything else, you might want to check out Degenatrons' XBox build
here. It includes videos and lots of details.
Scott