I've got an O'Day 25 with a single cylinder Yanmar (1GM1) and I've had good luck with it. Check the total hours and the maintenance records. I would suggest that you have someone familiar with marine diesels to look at it for you. In addition to the issues already noted, inspect the hoses and fuel tank for any evidence of leaks. I had a fuel hose leak that dumped 2.5 gal. of diesel into the bilge this spring before I caught it. If I had just inspected the hoses, I would have prevented the problem. It took a while to get the smell out of the boat. Overall though, these motors will give you a lot of years of steady service if you treat them right. Plus, if you get in any big waves, you don't have the same cavitation problems that you would with an outboard.
Good luck.
Dave
s/v Lagniappe
O'Day 25