AFAIK, any Johnson 8 HP Sailmaster will have a charging coil. Its rated at 8 amps, and should make 5 amps above 2000 rpm. I personally always felt Mercurys were among the best motors, but in sailboat motors, Mercury hasnt really offered a true sailer, leaving a sailmaster the only true choice, least wise if you live in NA.
Four strokes are okay, and if you live somewhere two strokes are banned you obviously have no choice. But they are much heavier for the same HP, are much more complex, and are much rougher (shake/vibration) at low speed compared to a two stroke.
A two stroke twin fires twice every revolution, and with the two piston throws 180 apart make the engine inherently balanced. A two stroke twin will idle down quite slow while remaining smooth. A four stroke twin fires once each revolution, and with both pistons going up and down together, they are inherently unbalanced. This is why we have all the threads about needing stronger motor mounts and beefed up transoms in regards to four strokes. They are shakers.
My personal choice would be a good used 8 hp two stroke Johnson sailmaster (making sure it has the charge coil, if thats important), followed by a good used Honda 7.5 extra long shaft. If I were buying new, I would only buy a 4 stroke because sooner or later all 2 strokes will be banned, and it would be a Honda. All Honda's are four strokes BTW, they never made a two stroke to the best of my knowledge.