what’s the difference between a babygro and a sleep suit?
No difference.
Do I put a vest underneath these? Or is it just nappy, babygro, cardigan (if needed).
Depends on weather, vest normally required in colder months.
Can I just stock up on the onesies / babygros / sleepsuits? These are ok for public right? I don’t need ‘outfits’?
Don't need outfits unless you want them, and lots of people will buy you outfits especially if it's a first baby.
what are bodysuits?! These seem to come in short sleeved and long sleeved. Wtf do I put on the bottom half? Do I need these?
Bodysuits are really vests with studs to stop them riding up. I never used long sleeved ones only short sleeved. Both babies were winter babies.
When I’m out and baby is in the pram, would I put the baby in nappy + vest, plus babygro plus pramsuit?
Pramsuits look cute but you might find blankets easier to whip on / off if you are going in and out of shops. Blankets are also better in carseats.
Pramsuits must not be used in car seats. They make it impossible to get the straps tight enough but compress with force. I've seen videos of baby crash test dummies fly out of carseats in pramsuits. Not good.
Everyone seems to love muslins. What do you actually use these for?
Wiping up puke, and putting over your shoulder to keep you clean.
At night I’ve seen the sleeping bag things and the tog is based on temperature. Surely the temp of the room fluctuates through the night so what do you do?
Just don't let baby get too hot. See the information posted up thread.