With DS1, I bought hardly any newborn stuff cause I thought he would be big. Nope

So I had to go out a buy a few outfits when he was born.
But he lived in onesies and gowns for the first 2 months. We just threw on some pants and socks and he had a little outfit. It made it super easy to change him.
As for the diapers, we bought one newborn pack at a time. It seems like they grow overnight into the next size. Once they get to size 1, you can buy more at one time.