Torsen or Quaife LSD. Will add about $600 to your rebuild. Get a Quaife from BAT or SP Motorsports and a Torsen from an authorized Ford Racing Parts Dealer- just don't expect them to install it.
Also, get the revised shift tower from the Focus,
search for the part number.
Get the updated shifter forks, search for those numbers, too. Cheap. Practically necessary.
And finally, find someone who is selling an extra Steeda short shifter + spacer that they might have laying around. $105 shipped if you find the right people. This one isn't necessary, just more fun.
*Oh yeah, as to what breaks.... IIRC, most cases involve breaking the diff.