Sinmama,
First, u need to buy the insurance b4 u can start to do anything..I dun think u need to wait for the Mom letter to buy but the insurance agent will ask for it so that they can transmit it to MOM directly and u can do the renewal of the WP online the next day. That what agent told me.
Second, rem to buy the additional Performance Bond beside the medical and security bond. When u first employ your maid, u dun need this but when u renew, the Embassy need it.
Third, if u wait for the MOM letter by post, normally it will be between 6 to 8 wks. But exactly 8wks b4, u can go to WP online, login using emplyer singpass, go to renewal and print out the letter.
Hope this clarify.