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There isnt a specific amount but I would expect at least 12 months bank statements. Expenses should go towards care and support of your father's day to day living. Again there isnt a specific period but family support should be for a reasonable period (in my view should be between 6 - 12 months at least)
The support should come from your husband or a joint account. If it comes from your account but your husband makes payment on your account, you would need to provide all statements showing the trace from your husband's account to yours and yours to your father.
Yes your father will be able to work as soon as he arrives. After arrival you father can apply for a Residence card with the same reasons qiven for the original EEA family permit. To fulfil the dependancy requirements your father should live in your household