A really thick, waterproof winter coat that she can wear light things underneath. The trouble with 'layers' for small children is they frequently don't take anything off - ds was often bright red in the face when I picked him up as he would be wearing 3 or 4 layers in an overheated school. IMO it's better to get a super thick coat as a top layer.
Closed shoes that she can put on by herself. You'd be amazed how many children have unsuitable shoes on at school. Don't give her any lace-ups (including trainers) until you are sure she is completely independent at using them (let's face it, few people buy lace-ups any more except for some trainers and Converse, but that's an embittered ex-TA speaking who spent what I felt was too much time helping children tie laces).
I personally am not wild about tights in school, fantastic though they look, unless your child is independent enough in dressing to be able to change her own tights if the feet get wet. But I don't have a daughter so I am willing to be told that's not an issue.
Waterproof gloves that attach to the coat. There's a reason that you still see children with gloves on a string going through their sleeves, it's the only way to avoid going through about 15 pairs a year.
I always regretted not buying snowboots when ds was younger. DS only wears them a few times a year but they make such a difference and you can resell them. Alternatively, buy decent wellies (i.e. that come up near the knee) and the fleece socks that go inside them to keep feet warm.
Likewise, waterproof trousers. No need to start the day soaking wet, anyway.
Get an easy way of naming things (like an indelible marker) and name EVERYTHING.