In hospital, button up PJs (or nightie if you have a c-section) are essential if you are breastfeeding, as you'll want easy and frequent access. Size - definitely a few sizes bigger than usual, your breasts will be larger than they are right now and for bottoms, well you could prob stay in pregnancy bottoms for a while if thats easier. Its going to take weeks for your tummy to go back to normal, so a lot of people still use their pregnancy wardrobe during this time.
A nursing bra and breastpads are essential, or if you're worried about being hot you can get nursing vests that you can stick breastpads in (www.bravadodesigns.com) for example). Get pads that stick in so they stay in place - johnsons are decent. You will probably get through quite a lot of nursing pads once your milk comes in but I wouldn't buy loads of washables in advance as they dont work for everyone, so get a few and see how you get on. Re size of nursing bra, I'd get a couple of cheap, slightly too large ones for early nursing (so you've got room both for very full breasts and nursing pads) and then (assuming you plan to bf for a while) invest in some nice ones after a couple of months when your supply has settled down and you can get properly fitted.
Pants - ditto Dragonbutter's advice. I found a pack of boring M&S maxis did the trick for me. But don't do what I did and buy the wrong size and only realise when in hospital (I'm a 12, I somehow picked up a pack of size 20s!)
Re baby clothes - have you looked at Grobags? Baby sleeping bags, come with a room themometer and instructions as to what the baby should wear under the bag at what temp. more here
Quick note re breastfeeding - some people have a very easy time establishing breastfeeding, others don't. I strongly suggest you prepare by doing some research beforehand (I didn't first time and regret that now!), and make sure you have some support line numbers with you as not all hospitals have BF counsellors, and most midwives are not trained in BF support and can actually be unhelpful in this area.