Any of the major autopilot manufacturers will work, but since you have a modern Simrad chart plotter, stick with Simrad for the autopilot. That will let you easily interface with the chart plotter so that you can navigate to a waypoint or course automatically.
You will need several components that are sometimes sold as a package like this one: AP2403 VRF Autopilot Package - Simrad Marine Electronics
You will need a display head, a course computer, a hydraulic pump, a fluxgate compass and a rudder indicator. Make sure that you get a gyro feature which will make holding course in a following sea much better. It isn't clear that the Simrad package above has that.
The hydraulic pump is plumbed into your existing hydraulic steering system. It will work from either helm. Since most flybridge trawlers are steered from above mostly, mount the display head at the upper helm and use a remote handheld for the lower helm.
This is not a simple plug in installation. I would find a local marine electronics shop who can put together a good system for you and install it.