The ST1000 is a bit slower and has a different, weaker, drive than its big brother, the ST2000. The ST1000 spec says it will handle boats up to 6,600 lbs. The O'day 25 is something like 4,800 lbs so the ST1000 is in spec for the boat.
Still, when I had my O'day 25 I bought the ST2000. It has a better drive in it, faster side to side movement and is designed for boats up to 10,000 lbs. After researching in the forums I felt the ST2000 sounded a bit more robust.
A search in these forums for the ST1000 and the ST2000 should give you an idea of whats preferred.
I have no experience with the ST1000. But guys who have had both have told me the ST2000 is significantly superior, and is the way to go even on a 22 footer, let alone a 25. Even the ST2000 (which we do love) is not what I would call a "robust" piece of equipment. I wouldn't want a cheaper model.
My OB was on the port side, I'm right handed, I frequently sat on the port side, my wife mostly sat on the starboard side. So it made sense to me to install the ST2000 on the port side.
Attached is a photo of it in use.
I have an oday 25 and have used both the st1000 and the st2000. The st1000 is not designed to stop when reaching the end of the drive. Over time it will reach the end, keep trying to move the piston and grind itself to death. This happened to me so I upgraded to the st2000 as it handles the end of the drive by stopping. Mount the unit on the port side to allow access to the engine and hatch when the unit is being used. The st1000 and st2000 mount exactly the same. My mount is like that shown by Ward H.
I would go with the port side also. Does the ST get in your way when operating the engine? I see the need for the bracket. Lucky you with two hatches. I have a transom vent that I put my gas line through. 2000 seems like the way to go.
It did a little. My Yamaha had the shifter on the tiller handle just behind the throttle grip so it wasn't too bad to reach over the ST.
I had always planned to put another socket on the starboard side but never got to it.