Well. Apart from the expandable bag, you'll want spell books. And like you said, food (an endless pantry, if you can get one) and a tent for your team (and make sure it has a proper bathroom because no one wants to have to go all that time without the chance to bathe) and a source for fire and water in case you can't use your wands for a time. And a cooking kit in case you can't get the endless pantry. Bedding, and maps, and brooms, of course. And extra clothing for everyone. And maybe books about the area you're in, if they'll let you have them.