I've not seen anything about her having been sunk. I think someone read about one of the two engines having been pickled and just made that assumption.
First thing to do is contact the seller and have him send you copies of the surveys -- the surveyor(s) will have taken all of the appropriate photographs to get you a ballpark estimate on refurb. Obviously the costs will depend on what your goals are for the boat, but previous posters have guessed in the low-to-mid six figures.
An earlier poster noted that the double-planked Alaskan Cedar hull would not need caulking. No worries about blistering either :-)
Just noticed that the current posting says nothing about pickling -- it just says that one of the main engines is running and one of the two generators is running.
More info here:
The Scoop Deck » The “ultimate” Naval Academy patrol boat could be yours