What are the takeoff and landing distances of motor gliders?
You may wish to obtain POH data from the 18-21 meter motor gliders you have listed and compare it to your calculated results. Also look into information such as wing loading, aspect ratio, takeoff speed (or stall speed), and airfoil type in addition to power source to include in your report.

Motor gliders, true to their name, should lift off well before 500 meters.  Landing distance would be similar and also available in POH.