living in Warsaw during studying

I have a strong opportunity to obtain a PhD at Warsaw University by the beginning of 2018.
Therefore I have several questions I wonder if someone answered it .
1. How much the average of monthly  expenses beside Warsaw University  .
2. Is it easy to find a part time job , in Engineering field , particularly architecture . if yes what is
3. In case of bringing my children (elementary stage) is there a Tuition fees in schools or they join a school for  free.
4. Does English  language commonly used or I have to learn Polish ?

Thank you
Ahmed

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