M_Boats: M-15 Ballast

I have been considering the purchase of an older (early-mid 80s) Montgomery 15 but have been put off thus far by some of the reports of problems with the steel ballast. Can anyone help calibrate these concerns by saying how widespread and how severe these problems are? Do they occur in all steel-ballasted M-15s as they age, or only those that have been kept in the water most of the time? Does it represent a minor inconvenience that can tolerated, or a downright risk of the keel coming apart? Are there measures that can be taken to prevent or remedy the problem, short of major surgery? Your collective wisdom would be appreciated. Thanks. The new guy.