A soft starter is only good if you run at full speed. If the speed is variable you can only use an AC drive and the saving is higher the lower you run the motor. By law you have to use a IE2 motor or better from June 2011 when the motor power is over 0.75 kW anyway, but regard the load the best saving is always obtained by correct sizing and choosing of the right components, so you run the close to nominal speed as possible.

Soft starters also have a limitation on the number of starts per hour. In this application, there would typically be 20-30/hr.

Regarding AC drives, three steps are needed to protect the motor:
1) High enough winding voltage withstands voltage (dielectric strength),
2) Adequate thermal capability to counter the extra (5% or so) winding heading due to the harmonics, and
3) protecting the bearings from developed stray voltage (grounding, bypassing or insulating).