C28 Autopilot
I can't tell what you consider a standard Autopilot, or what you're considering. Since Maine responded with a reference to the S1, I thought I'd chime in with my experience because I'm doing similar research right now for my Catalina 28 mkII, and found a decent price on the S1. A more relevant question might be where to mount the S1 sensor arm for your C28. Is it a mkI or mkII? From what I can tell, the position of the rudder quadrant is essential to the S1 rudder position sensing, and the autopilot mechnanism must be attached to the quadrant. I believe that on the mkI, the quadrant is located below the deck, where it is accessible through the aft berth bulkhead, and this is doable. On the mkII, the quadrant is housed in a shallow well that is only accessible from the deck (and otherwise sealed from access below).
For what it's worth, I've heard that the newer model X5 Autopilot does not require a sensor.