Hi, most mums I know liked different things, found some stuff more useful than others etc, so this is just my opinion, I have a 3 year old and a one year old.
I love grobags, you'll need 2 so you can be washing 1 whist using one, smallest size, summer weight (1tog). You do not need to use any blankets when using a grobag.
I found I needed loads of short sleaved vests, they'll constantly be getting covered in poo, wee and sick so you'll need enough for them to be changed a couple of times a day. Guess it depends how often you want to be putting the washing machine on.
Personally I find all the poppers on romper suits annoying, especially when changing a dirty nappy at 2pm. I like those long sleeved nighties with the drawstring bottom at night, long sleeved t-shirts and comfy elasticated waisted trousers during the day, both worn over short sleeved vest. If it's a warm day you can just remove the long sleeved t-shirt.
As for a hat, guess it depends on the weather. If at all chilly I would put a hat on a newborn, and if sunny you need to protect with a hat, or better stil keep in the shade.
I never bothered with shoes, they don't stay on and apparently are bad for their feet, little socks will do. Would avoid stuff with built in feet, it saves on losing socks but they grow out of them much faster.
You'll need a couple of cardigans easier to put on and off than jumpers.
I never bothered with outdoor coats/all in one suits, unlikely you'll need them in summer and far easier to wrap a newborn in lots of blankets that you can easily add or remove when going in and out of shops etc.
I had one of those cosy little covers for the baby carrier.
I would also buy loads of muslin squares, can be used as burp cloth, to wipe up spills, makeshift bib, very useful.
So, in list form:
10 short sleaved vests
5 trousers
5 long sleaved t-shirts
5 pairs of socks
3 drawstring nighties
2 grobags
4 baby blankets
10 muslin squares
3 cardigans
1 hat
Hope that helps, sure you'll get lots of conflicting advise.
Good luck Sxx