North Star 727
As I said before these are cool little boats. I can''t recall the interior layout on the 727 for certain but if I remember correctly there is nothing but the head forward of the bulkhead (maybe a vee berth that was out of the boat that I am thinking of) so you should have good access to the forward face of the bulkhead if not the aft face. Often you can only replace the rotten portion of the bulkhead and laminate a piece of plywood on the other side of the bulkhead to improve itrs strength. Replaceing the entire bulkhead is not an easy job on a finished boat. Bulkheads are usually installed before the deck and interior are put into place. Still an amatuer with reasonable woodworking skills should be able to do this successfully. If all that is rotted is the block of wood under the mast itself that is not a bad job to do. BUT if there is a wood beam and it is rotted that is much harder since you will need to either replace the beam or do a long scarf joint, neither of which is the easiest job to do.
My big fear on those boats would be the keel bolts and keel fairing material. If all else looks solid and you are just talking about the bulkheads and mast step, I would probably follow your friend''s advice. If all else is in good shape, at $2500 and even with the bad bulkheads, this is not too bad a deal.