I wouldnt be quite as concerned with hours. Rather i'd do a compression check and see how well the motor was taken care of. Unless the newer motors are different...if taken care of properly, a boat motor can last a very looooooooooong time and virtually maint. free. Buying used is ALWAYS a gamble so dont go solely on the hours an engine has been ran.
Health nuts are going to feel stupid someday, lying in hospitals dying of nothing.
Where I live fishing is the 2nd biggest industry and farming is # 1. I commercial fish for a living and any motor around a 75 to 100 mile radius with a lot of hrs most people try to stay away from. If a motor here has 1500 to 2000 hrs on it that is not that much for as many hrs we are on the water. IMO I would want to no how long it has been since it has been cranked #2 look at the apperance of the motor,look to see if you see grease near the lower unit, that ant good,#3 look at the prop to see if it is bent up that might tell you how the person uses it,#4 if you still like what your looking at ask if you can take it to your mechanic not his to do a compression ck. If he says he would rather not let the boat out of his sight I would stay away from it. One other thing, IMO I would buy a 2 stroke,I've owned both a 4 stroke Johnson and 2stroke johnson. I promise you if you have trouble out of a four stroke you are going to pay dearly. Just a carb problem is very pricey and the parts for it are just as expensive,not counting the labor. I currently own two ETEC's and are very good on gas and oil. I hope this helps and I hope I didn't offend anyone,I've just been taken before because I trusted people.
Good luck