To start 55kW motor with 300% starting current limitation 1

The invertor is usually the output stage of the soft starter. This is the stage that generates the variable frequency & voltage used for soft starting the motor. This needs to be coupled with a rectifier stage and with a bypass to provide the complete starter.
Another option that you may consider is an autotransformer starter, which should be less costly and also require less maintenance - although this could take up more floor space than the soft starter.

An inverter is a variable frequency drive that takes power from your standard AC bus, converts it to DC and stores it in a capacitor bank, then uses a computer-controlled switching network (often IGBTs) to create AC (inversion). The parameters of voltage, current, frequency, etc., can all be controlled (within limits) based on the stored program in the inverter's computer. Think of it as an intelligent soft start.
Yes, it can perform the function you are seeking. As far as space goes, you'll have to check out products from different vendors to determine the sizes available.
Make sure you become familiar with the application before you select it. Inverters create harmonics by the very act of inversion, and these can reduce the life of your motor in some instances. Many motors are designed specifically for use with inverters to overcome this. Be sure you understand the limitations.

The invertor is usually the output stage of the soft starter. This is the stage that generates the variable frequency & voltage used for soft starting the motor. This needs to be coupled with a rectifier stage and with a bypass, an isolator and overload protection to provide the complete starter. Suggest that you get the proposed vendor to provide you with technical details and/or shop around some more.
Another option that you may consider is an autotransformer starter, which should be less costly and also require less maintenance - although this could take up more floor space than the soft starter.
For a useful overview of soft starting, see this reference:

More info about your motor could lead to an optimal solution. E.g. voltage, number of phases, and stator connections. For example, if you have ends of stator coils available in the motor terminal box, then the y-delta hand-operated 3-position mechanical switch may be a reasonable, reliable and inexpensive solution, if the Utility Co. approves it.

i can help you. i have the knowlage about this topic.