I had a 7lb7 baby and 0-3 was WAY too big. We had 3 x vest & sleepsuits in newborn and 3 x each in 0-3 in my hospital bag. After one night I had a pile of sicky clothes, a baby in what looked like a pillowcase it was so large and was texting DH saying "find more clothes!". She'd spent her first night puking up amniotic fluid, wasn't actually that pukey after the first couple of days.
I think we used newborn size for about 3 weeks and then into 0-3. Would agree with about 5/6 of vests & sleepsuits in newborn and maybe a few more in 0-3 (as they'll be in use for longer), but you can buy those when you need them. My friend had a 6lb baby and newborn dwarfed him so she sent her mum for tiny baby size. It's hard to judge beforehand though.
I breastfed and would recommend lansinoh (1 tube is tons) and lansinoh breastpads (off Amazon, much nicer and far more absorbant than the asda/tesco etc ones I tried), and lots of muslins. I had to sleep on a towel for a while due to leaky boobs, and DH did get sent out for a manual breastpump on about day 4 as I was so engorged my nipples practically disappeared and DD couldn't latch - needed to pump and dump a bit.
You need nappies (just get one pack of newborn size, they grow so fast and one pack is enough to get started), and cotton wool or wipes, nappy bags (cheap ones are fine). Change mat and a bag to use as a change bag.
Carseat and pram/pushchair/sling depending on preference.
Cellular blankets, cot sheets, waterproof mattress protectors.
We just bathed DD in the sink when she was small and the bath when she was bigger, didn't use anything other than water for ages. Didn't worry about toys/playmat etc until she was a bit bigger, newborns are not interested in anything other than being held/feeding/sleeping.