i dont't think avent has the bag. you may want to get the ones from Pigeon for storage (available in some supermarts, robinsons, OG, kiddy palace etc). or you can get Avents Via cups - plastic and reusable.
if you intend to use only part of your expressed milk, then separate the amt you want to feed first before feeding as storing leftovers from the feed may contaminate the milk. then the other portion meant for storing can be put in the fridge if you intend to use within 24hrs. else freeze it.
how much to express depends on how much your baby drinks i guess. if the baby finishes whatever you give him/her, then try increasing by 10ml the next feed to see if the baby can take it.