Sounds like owners neglect, engine needs oil every 100 hrs min!! I do mine every 75 hours. If you let it go longer the oil viscosity really breaks down fast and could cause issues. Its possible there were other reasons why this happened but seems out of the ordinary. I have just under 900 hrs on mine, they are 2002's. We bought the boat in 2004 and put 760 of the hours on ourselves. Not an issue with oil ever. Not sure why there would be an oil problem and power head problem on such a low hour engine, something either seems wrong with that engine or there was serious neglect somewhere in its past. Have it looked over by a Yamaha certified tech, engine could have other issues if it was neglected, or could be a great deal, only a true certified tech can check it out thoroughly and tell you. They should be able to get computer read outs that can maybe explain why the engine had issues, might show a rise in temperature, high rpm usage or something else. Have it looked over, could save you thousands in the long run, or it could confirm its a good deal and great buy for you.
