There's a metric crapton of fridge builds in here. Time to go searching!
Look for "kegerator" or "fridge"
You're going to have a hard time finding a dorm fridge that will support a full 1/2 bbl keg. You might have to look at an actual Haier or Danby (Or any of a number of other kegerator manufacturers).
I was going to convert a dorm fridge, but found a Haier kegerator on CL for $200

Then I converted it to 2 tap and upgraded the faucets to Perlicks.
Hunt around long enough and you will find a good deal. It just takes patience.
If you want fast
and cheap, you can always convert a full size fridge. They are really easy to find on CL for next to nothing.