Hardest part of any IO is checking that the transom and stringers aren't water damaged. Other than that - do a sea trial or pass on it. If you can run it around for an hour or two and it still sounds and works right, that is a great deal of the angst involved. Then, after a goodly long run, pull the bottom plug on the drive, let a couple drops of oil out - no water contamination?
If I knew what I know now when I bought my boat I'd have taken my vacuum oil suction unit with me after the test drive and pulled a teeny bit of oil off the bottom of the pan after the test run. See if there are any contaminants there. I didn't have a water in oil problem when I bought my boat, but SO many do have cracked blocks that I can't help but point out how cheap a vacuum pump oil changer is and how fast it is to drop down a dipstick tube and pull a sample.