Which motor should I use to control the steering wheel?
You should check some of the suggestions for motors listed here.
the motor · farmerbriantee/AgOpenGPS_Boards Wiki · GitHub
I suggest using one of the more popular models.
The most popular is the phidgets 24v motor. The 3d printed cogs and motor holder are designed around this motor with lots of info and support on it. There are of course lots of others available aswell some 12v options
What speed should the engine have?
The link provided lists 3 motors known to work well and their speeds are on the same page. A motor with similar specifications should also work well. 600-1300 RPM, 3.3-5.1 kg-cm of torque. Anything with kind of similar specs should work. Slow motors will still auto steer an AB line well, they just won’t do a U turn.