Requesting Advice For Selecting And Sourcing A "Medium Torque" Motor
When choosing a motor, thinking about torque first is not the way to go. As you have pointed out in your question, you can vary the torque simply by varying the size of the winding drum. You need to worry first about the power of the motor. This gives you the weight, cost and power supply requirements. Then an appropriate gearing will give you the right speed and torque.
If you want to pull 2 kN at 0.5 m/s, then you need 1 kW mechanical output, which will require a 1.5 to 2 kW electrical input.
The line speed should not vary with the applied force? All motors, except synchronous and stepper motors, will vary speed when under different force loadings, but you're unlikely to be using one of those(#). Permanent magnet and shunt-wound motors will change a little, series wound motors will change a lot. Either type of motor can use feedback to control the speed to any given degree of constancy.
If you are going to control the position of the cable with a computer, then you will first need to sense the position of the cable. There are lots of different ways, including counting the turns of the drum with an angle sensor, or measuring the distance from crane to hook with an ultrasonic sensor. Different ways have different accuracies, and different ease-of-use, or requirements for setting a zero. Anything that uses motor or drum position is subject to cable winding errors on the drum. What position accuracy do you want, ±1 mm, ±1 cm, ±10 cm? The amswer will make a big difference to what techniques you can use. Ultimately, the position feedback can be used with your computer to control the motor speed and position.
(#) A BLDC motor can be used in stepper-motor mode or with a full position controller, and this would give you direct control over its speed and position. You might find one powerful enough.