Well done for thinking of everything in advance!
I'm not going to be of much direct help in answering your question, but just wanted to point out that formula milk is readily available in late-night opening stores, so that if you decided to supplement or quit breastfeeding you would be able to get hold of some easily.
Many women who intend to breastfeed prefer not to have any formula in the house, in case it is too tempting to give to baby if you have any breastfeeding difficulties!
Once breastfeeding has been successfully established, some families manage to successfully combine breastfeeding and formula, but before breastfeeding is established then formula feeds encourage a downward spiral of breastfeeding problems. I believe that with the right information, support and encouragement the vast majority of breastfeeding problems can be overcome relatively easily. There are lots of pointers in the Feeding Baby section of this forum!
With my two babies, they have only had formula after 6 months (which is a different type of formula) and my criteria have been to find a package which is not too big, because I didn't plan to use it much, in packaging more biodegradable than those giant thick metal tubs. There are some brands such as Hipp Organix which have boxes with foil packets inside.