Thinking about getting an older boat

I am looking into buying an older sport Nautique, something around the early 90's. I am trying to figure out my finances, and am going to set up a savings account for boat repairs and such. I am trying to figure out how much I need to set aside- and am trying to figure out some worst case engine repairs that could pop up in a year or so on an older boat like this. How much would you set aside for something as serious as an engine rebuild, transmission problems, or replacement on either if needed. How common are these problems on an older boat that has been taken care of?