You have had lots of good answers already so will just add two quick thoughts.
– the way that the autopilot motor has been fitted on many D34 and D40s causes water from the cockpit drain to enter the housing of the autopilot, resulting in a total failure of the autopilot which jams the steering. I attach a screen print of a French forum relating to this. It’s worth checking this when you install the autopilot
– like many D34 owners, I have a Raymarine series St6000. A review of the Dufour forum will show you that a number of people (including myself) have had persistent problems with it. I don’t know whether any other models are different I’m afraid.