I have one and a few other guys on here do as well.
I can say this, generally surface drives are designed for "mud" hulls, which are smooth. If you run a surface drive on a ribbed boat, you are not going to achieve the full potential of the motor.
I am running my SD on a riveted alumacraft and it does ok, but a mud hull built specifically for the motor I have is on my list. I do get into places you wouldn't go with a prop motor, but I have not run my boat in the river yet so I can't comment on how it will perform with a hard bottom.
As far as where to buy them used, your best bet would be to check out mudmotortalk.com That site will also answer a lot of your questions you have about surface drives.
does anyone have a surface motor. i waas thinking of getting one for hunting and fishing. just wondering what eveyone thinks of them. pros and cons? is there anyplace to buy them used.
Runnin' on the river? I am not experienced with mud motors. But, IMO outboard jet drive is the way to go if you can find one small enough for your boat's HP rating. Only drawback is that jets don't like weeds. They're basically a water vacuum. Oh yeah. One more thing. If you buy a jet, make sure you mount it properly. The lip of the intake must be even with the bottom of the boat. A little high or low and it will either plow the water or suck in air. I used to run a 16 foot Roughneck as shallow as 5-6 inches going flat out with a 40/28 Yamaha. Good engine. Bought it new in 1997 and it only ever took a water pump and several re-shaping the intake tines. Still have it. Starts every time.