Where you have the cord between the lever and the door handle put the spring balance suggested by ~ChrisTenone. Then open the door by pulling the lever by hand and measure the maximum force needed.
You then need to multipy that force by the distance from where the cord attaches to the lever to where the lever attaches to the the stepper. That will give you the torque the motor needs to have.
The way things are setup the lever is giving mechanical advantage to the door, it is working against the motor being able to move the door. Try putting one finger on the lever far from the stepper and push the door open. Now do the same with your finger half way down the lever. Then try it with your finger near the stepper. more and more force is required the closer you get to the pivot point.
The idea of a pulley suggested by ~Travis sounds like a good one to me.