1. Monthly plan without HKID (residency ID) card requires cash deposit. Last time I checked a few thousand HKD . Prepaid plans are cheap enough.
2. You buy a phone, full price. You get a contract, and they give you monthly "rebates", thus they advertise "zero dollar phone" "0 機價".
3. Basic phone starts at probably 300HKD.
4. Basic phone and sales advice, I wish you good luck. Sales people are impatient to begin with, except at the Apple Store, or brand showrooms - not that many around. Network provider boutiques might be OK too.
5. Perhaps check with local message boards, such as phonehk.com , discuss.com.hk . You can post questions in English. They often discuss basic phones for grandma and domestic helpers.