We had a 1997 350 for about 10 years - Great boat. If I were buying another one ( or any IP ) I would check the bottoms of the bulkheads in the head. If the shower has been used a lot, the bottom of the walls in the shower rot. This is a common IP problem, not limited to the 350. Here is what happens, the water runs down the wall, under the teak molding, and then "wicks" back up the plywood behind the hard plastic veneer.. It looks really terrible when you discover it.. You can usually push a small screw driver or ice pick in the bottom of the bulk head. Not to worry, it is easily fixed, and doesn't seem to be a structural problem.
Open the wall by removing the bottom part of the wall covering, clean it out the rotten plywood, bond in a replacement piece, then cover it with a matching formica piece on top. If it hasn't happened yet, Remove the teak trim at the bottom, Caulk it with 5200 or something like that to stop the problem, and replace the trim. This was a problem on my 350 and it was a problem on my IP40 in the front head. I would be willing to bet that many others IP owners have this problem and have never discovered it.
Good Luck
George
S/V Delicia