corrosion in the center section

I have had 2 different IA s warn me about possible corrosion in wing center sections. I also have read the pre-inspection guide by Ed Birkhead. I have found a 415d here in central California with metal wings that has been kept here and hangared alongside the owners other King Air and 2 Bonanzas for the last 30 years. I am getting ready to make an offer on the plane but am wondering what the percentage of coupes in dry climates is that would possibly have this problem. Apparently there can be undetectable corrosion in the center section even with the 16 inspection holes in the wing bottom??? Any advice would be appreciated, is this something to really be concerned about given the location of the plane. It never gets wet, never sits outside..Thanks