Personally I always used:
X5 newborn vests/ and sleep suits
X10 0-3 vests and sleep suits
That's enough as newborn size generally first few weeks only max. If larger baby even less. Then I figured if baby really puked/ hadn't washed the newborn ones they could easily wear the 0-3 just baggy until washing caught up, but can't say we needed to. If you have a really puky baby and using all 5 every day, then can easily buy more after.
I also just brought plain white babygrows in newborn, 0-3, and 3-6 month size. Meant all can go in same white wash, can wash high temps and stains comes out if any. They are cheaper also. ( but still about £100 just on babygrows to last newborn-6months). Then can buy nice cardigans/ hats and booties to go with them.
Nappy wise I would by more than say 100 of each size. As might change sizes quicker than you think. Although have you considered reusables? The one size fits all last newborn- potty trained, and just popper smaller/ bigger than on like normal nappy. About £300 will get you 25 of them. Very cost effective, especially if you have another child ( think its £1600 average cost of disposable nappies per child)
Amazon is good. If you join Amazon family you save up to 20% I think, and next day delivery, so good if you suddenly want more of something