The 65hp 4-cylinder was a decent motor in its day, but it was kind of an odd duck in a way as it was the least "modular" of all the big Mercs. Mercury was fond of stacking cylinders to produce bigger motors, .ie the 2-cylinder 40 was 1/2 of the 4-cylinder 80, and 1/3 of the 6-cylinder models. The 65hp really didn't have that benefit, and as such it had a lot less parts support.
Bottom line is if you can get it for a good price and are willing to take the time to track down parts, etc., you may end up with a good motor. If you're not very mechanical and don't have the patience or ability to work on it yourself, take a pass on it...