I just have finish installing an AP12R rotary cable autopilot from Simrad. The GPS interface is NMEA 0183. Bought it from BOE for just under $1000, free shipping. The manual can be downloaded from Manual. The manual is very good in explanation of removing existing and installing the autopilot helm.
Most small boats have rotary cable steering systems and the next most popular steering is hydraulic. There is only one autopilot for rotary cables systems and that is the AP12R, that I’m aware of. There are a lot of hydraulics and a few other mongrel types for sail boats.
The NMEA 2000 is not compatible with NMEA 0183, if you think that you need the 2000 features then you could get a Translator. Your GPS will give you steering info on one of its display pages, with that info you can perform the necessary maneuvers to steer to waypoints. The AP12 has a feature where you can adjust heading in 1 degree increments for manual coarse adjustments. With the GPS, you can still setup routes containing multiple waypoints and just manual steer to the new waypoint, good to use in fog to get home, etc. 
