Further JAWZ's post... If you are going to sea trial a boat, make sure that the engines are COLD when you get there.
Tell the seller that you want to see the boat with cold engines.
When you get there, pop the hatch and feel the engines...make sure they are COLD. Lots of problem engines are major pains to start cold but are pussycats to start once warmed up.

If you aren't going to sea trial a boat, deduct the price of a new factory engine from the current market value of the boat in that boat's current hull condition. Don't pay any more than that. If its a stern drive... factor in a rebuilt lower unit as well.

At best, boats are "holes in the water..." etc.etc. No point in making it any worse than it needs to be.

If the boat is more than a few thousands... get it surveyed,