I would get 2 boxes only, and one of the next size up. It is impossible to tell beforehand how long they will be in each size for. If you are worried about money to buy them when the time comes, then just put some money away each week so you don't have to find $40 each week or so. My lot weren't in them for long - my boys were out of them by 2wks old but the girls were 4-5wks old. DS1 quickly moved out of the next size too and was onto the crawler nappies by 2 months old.

Huggies have changed their sizing now so the newborns ones are up to 5kg, infant 4-8kg ifthat helps at all.