<<Can any one on the list tell me what the differences are(design and
  fabrication wise) between a 24V starter / 24 Alternator and their 12V
  counter parts.  I purchased an IO 550N with 24v components and I am
planning
  a 12V system for my Legacy.  The components have 17 hrs on them so I would
  like to reuse if possible.>>

I'm sure B&C will also tell you that the alternator will work just fine on
12 volts, but if they wound the field differently there might be slightly
less output at idle.  The starter, I'll bet, will crank the engine a little
slower than a 12-volt design.  If I were you and trying to save a little
money I would try it out - everything will probably work okay.

Gary Casey