Regarding the 70's "Johnson Sail Master"...
just my own biased opinion but I always thought that it performed like about a 7 hp engine with a way-cool sailboat decal on the cowl.
That, and those two stroke motors only developed their advertised hp at their highest howling rpm. IMHO none of 'em were well suited to sailboat hulls.
I used to motor a 4000# Niagara 26 around with a Yamaha four stroke "Hi Thrust" 10, with a 3 to 1 reduction gear. As outboards for displacement hulls go, it might as well have been from another planet -- the planet of max torque. It was wonderful.
As for diesel vibration, it's true that more cylinders = more theoretical smoothness. But not nearly as much as legend would have you believe. My guess is that by the 70's small diesels had advanced their design, compared to the older designs from decades prior.
I did a delivery of about 250 miles on a '79 Yamaha 33 sloop with the original Yanmar single horizontal cylinder 12 hp diesel. It powered the boat along at six knots @ 24 hours at a time with no fuss and was only slightly noisier than the three cylinder in my own boat. It did have a well-insulated engine compartment, from the factory.
If the Ericson 27 in question has the correct pitch prop and is running well, I betcha it powers along at 5.5 or 6 kts in smooth water and then all you have to do is decide whether or not you are sufficiently in love with the whole boat to buy it!
One other nice thing about having fewer (or only one!) cylinder is that service is less expensive -- only one injector to ever rebuild or test.
Cheers,
LB