You have significant flexibility as to mounting location. I have mine mounted at the transom, with the display facing forward. You could also mount it at the companionway bulkhead, or even on the port or starboard seatbacks. Really, it's wherever it's most convenient. As with all such units, you do want it to be away from magnetic fields, such as a speaker. But that's about the only limitation.
The Pelagic steers my boat with authority, unlike my old ST-2000, which was more of a toy by comparison. Tech support is also good. I did have some hardware problems early on, but the replacement unit has been working quite well. I'd suggest getting the remote, which I find comes in handy. It should certainly steer a Catalina 22 with ease. Assuming you mounted your ST-2000 correctly (i.e., 18" from the rudder pivot point), the Pelagic actuator will work fine in the same location and drops in place. It does use a different electrical connector, so you'd have some very minor re-wiring to do. It would be a pretty quick job.