I would buy mainly 0-3 mths but you will need a few newborn sizes at first, my youngest was quite a big baby and 0-3 mths didn't fit her till she was about 6 wks so I would buy some newborn sleepsuits and body suits for hospital at least maybe some other bits too.
My first baby needed tiny sizes but I wouldn't buy them unless you knew your baby was going be very small or born early. Im going buy a few things in newborn and then mainly 0-3 mths.
As for nappies I would stock up on few packs of size ones not too many sizes 2 cos there actually only about 2lbs bigger than sizes one so they may out grow them in wk or 2, but buy plenty of size 3's they will last for ages. It is handy to buy some formula just incase you have trouble with breastfeeding and they don't usually give you formula in hospital now days I had a friend whos hubby had to drive around at night to look for open store to buy formula as hospital wouldn't give them any its also handy to use to top up breastfeeding if you don't have much milk.
Ive got a sterilizer, baby bath, a car seat in gr8 condition and moses basket so I don't think I will buy them new just all the other stuff got feeling im going end up buying 2 prams a normal pram for a baby and a cheap double i think will come in handy till my lo is walking more x
Also when buying clothes, shops come up different in sizes newborn is meant to be up to 9 or 10 lbs where as in next newborn is only up to 7.8lbs so be careful and check the weight as well as the size x