I finally made a choice of the motors to use. They are heavy, expensive and require special control hardware, but they will probably do the job. The motors are two geared brushless DC motors that runs on 24V. They each deliver 1.6 Nm continuously and have a top speed of 185 rpm. This means that I went for speed instead of torque, but it will be possible to replace the gears if the torque is not sufficient. With alternative gearing, the motors can deliver 2.8 Nm at 95 rpm and 4.1 Nm at 65 rpm.
Now it is time to figure out how to get the motors to run. Unlike ordinary DC motors, brushless motors need a dedicated control box. In the best of worlds, this box would contain an USB interface, but in reality all systems like these are controlled by a range of different analogue and digital control signals. The main problem with the motor controllers will be to generate these signals from a computer in a way that is not too complicated.
