I bought a used 2014 with a Ray Marine autopilot. Don't know if it was a factory installation. It is a complex setup, with many bits and pieces. I'm ordinarily a very much do it yourself person, but in this case I would at the very least go to a specialty marine electronics store and get a list of what they would recommend installing.
The autopilot is a great thing to have. After a while the constant small manual steering adjustments with the wheel get to be a drag - like a car with too little caster alignment. Mine has a hand held remote control so you can control the auto pilot remotely - cool feature.
A couple of comments on my specific Ray Marine setup: I would sort of expect an autopilot to have some sort of electric clutch to disconnect the manual wheel during auto steering. The Ray Marine setup has a fixed friction coupling between the manual wheel and the autopilot motor. With the auto pilot off you have to overcome the friction to steer the boat manually. The drag is pretty strong and annoying - like steering a boat with the wheel lock on all the time. Also, the steering motor is installed on the end of the original wheel shaft, with a new wheel shaft sticking out of the face of the wheel motor. This takes about three or four inches of space, and moves the wheel that much closer to the helmsperson's gut. God knows there's not a lot of room in the 21's helm seat and this makes the wheel pretty intrusive.
All that said, the autopilot really is a great thing to have, and greatly increases my enjoyment of the boat.
Larry