1) As far as I know, only the previous employment contract has the address of the employer, the passport just has the name only. You should get the previous employment contract and send a letter to the address and ask her previous employer to call you.
2) Up to you, at home or in agent.
3) If she is already working in HK, I think she has all the stuff. No need to prepare.
Don't let her know you are the first time to hire maid, she will take advantage on you.