Lars, Steve, the Wiki is invaluable with all the schematics and ideas that have been tested and tried - and importantly the ones that don't such as the UM7 or using the BNO for anything but heading. The Brick v2 is the answer if you want to use heading. Andreas has done an incredible job of collecting the knowledge in 1 place.
The mega is the same as an uno or a nano just has more extra pins, so all the wiki examples apply. Many people have built the autosteer with the DOGS2, arduino, and motor control board all together and it works just fine. To start out, all you need is the arduino, steer sensor, motor driver board and a steer angle sensor. And even at that it works quite well if you don't have too many hills or very bumpy ground. Definitely don't use the UM7, it drifts really bad.
Dairytech, I for the life of me can't remember why i used 2 sockets, but it had something to do with threading all the networking so send and receive could independently work in their own thread. What you may notice is you can connect and disconnect any module at any time and it just starts working again. That is also the advantage of having separate sockets. Good you got it working and yes, its all bytes all the time
The real bonus for learning ethernet and micro and C# this way - its also a useful project. HAving this in both 5100 and ENC28 is awesome. Anyone know if the 5500 is compatible code wise with the 5100?
I was thinking to day I should make an ethernet Steer AngleSensor just like the Lidar.