I just had my baby in December. She outgrew newborn size in two weeks.
I had 6 babygros and 6 vests in newborn size and that's all I needed.
I bought a lot more 0-3 and she never wore a lot of it, and certainly not more than once or twice. She was out of 0-3 by two months old.
In fact, she's 4 months old and already hovering between 3-6 and 6-9 on some brands!
It also means I have summer stuff that she'll never wear as if hasn't been hot enough for some dresses (and some I just can't be arsed to iron!) plus babygros are just so much easier!
I wish wish wish I'd not bought so much in sizes 0-3 and 3-6. (and in fairness, we received a LOT of gifts and second hand from family).
I wish I'd ordered what I needed as i needed it!
My point is not to stress! I'd just buy newborn for now and worse case scenario, you can buy everything you need at the supermarket during a food shop. Can't rate supermarket babygros enough. I love Aldi's babygros and vests (organic cotton, CHEAP! Pretty and wash well) and they've got a baby event on at the moment. I just ordered online from them and my parcel came within 3 days. Their baby sleeping bags are great too.
Also, same with toys and stuff like that. Won't need them for weeks and weeks so don't stress. I loved Ikea for platmat, gym, soft toys etc.