Small outboards almost always have an internal gas tank so you don't have to worry about that. There's usually a connection for an external tank too, which often comes with the outboard.
The outboard attaches to mount on the port side of the transom. It's trivial to put on. You cat put it on and take it off while you're sailing.
If there's much current you'll want the motor. Do you see people sailing on the river where you want to sail? Do they have motors?