I'd venture out on a limb and say that small motors (say around 40 and below) ought to be in a different category than large ones. Also, when I say old, I mean "old," not a used motor that has a 3 or 4 years on it - I'm truly talking '60s, '70s, '80s motors.
In small motors, the old ones are generally going to be much easier to work on, without specialized tools. Due to their simplicity, they are usually pretty reliable (disregarding older Mercury's - don't know what was going through Karl Keikhaffeur's mind), and have excellent longevity - no electronics that cost a fortune to wear out, etc. Their initial cost of ownership is usually a lot lower, which for many is the difference in being able to get on the water, or not. With the exception of the few remaining 2 stroke Yamadingers (I put this as an exception, as these aren't all that common, and don't represent the "general" line of new motors these days), the old ones are often lighter, but that is due to the 2 and 4 stroke difference.
However, they are going to be a little less fuel efficient. They aren't going to have what I like to call creature comforts. As in, many will lack power tilt and trim. They will often seep and leak a little, which is no problem, but sometimes it is annoying to get unburned oil on your hand as you lift the motor to drop it in the motor toter. Won't be as quiet.
On larger motors, the old has more cons than pros.
The larger ones just don't seem to last as long. Speaking in terms of Johnnyrudes, the small 2 cyl. motors will run forever and a day. The larger ones tend to wear themselves out quicker. They have simplicity, but I'm not so sure of the reliability gains.
The fuel economy is definitely worse, and they still lack the creature comforts.
Lets just say I would buy a '68 model 5 horse in pretty short order, if I needed one. I would probably not put the '68 55 horse on any of my boats, even if I got it for free.
When I finally (I'm not finding near as much work as I anticipated), get the money to build the 21 ft. alloy center console, lets just say I'll be looking for a lightly used (solely because I'd like to let someone else take the initial depreciation, I don't like to buy much of anything "brand new") Honda BF225, as opposed to an '80s Evinrude 225, even though the cost difference would be significant.