M_Boats: Core rot

Dear M-boat owners, the recent spate of e-mails got me very worried of core rot. How common is it? The problem is I am not very handy with fiberglass, and this seems to be quite a serious problem, especially in the older boats (I am looking at buying a 1975). Am I right in understanding that if the deck fittings have not been reseated, then the likelihood is quite high that there is a significant amount of rot? How many of you have had to redo the decks? Thanks, Andrei.