Autohelm/Raymarine made both an AH3000 and an ST3000. The only difference was the control head.
When I got my boat, it had an AH3000 control head, and I changed it to an ST3000 head (via an adapter lead) that had Seatalk. The motor was the same unit for both versions. I have the manual for the ST3000 on the boat.
I fed the GPS NMEA via an Interface Box to the ST3000 so that I could get the GPS to feed track information to the ST3000.
This allowed me to get the GPS to steer the boat on a track instead of just steering to a waypoint with the use of the ST3000 only.
If you search the UK ebay
, there are some AH and ST bits on offer.