I haven't used a Johnson Sea Horse since I was 10 to 15 yrs old. ( A LONG time ago ).
Direct drive. If running, you were moving. for reverse, you pointed the engine in the other direction.
If you are looking at an original, they do exactly what they were designed to do. And, as with any used engine, "how good is it" depends on proper care and maintenance and responsible use and care.
If you are looking at something "modern", I have never seen one. Who knows what you are getting.
In any event, in the interest of maintaining the engine, I would look to acquire something less that a decade old...
( If Thudpucker were still with us, he would likely have a few refurb originals you could barter for... )