The nursing bras that are meant to help you pump don't really work in my experience - I always have to hold the pump. Although I have a friend who swears that using one of those bras her sister was able to do housework while pumping at the same time. But I think she was winding me up!
For good nursing bras, I massively recommend Hot Milk. I wear a 32F and Hot Milk is the best brand I've found.
I had to buy more then once as your size does change. I had a couple during pregnancy as it was more comfortable to sleep in a bra. They kept me going for the first 8 weeks or so after ds was born. Then when your milk settles (around 8 weeks), so does the size bra you need - I bought another two bras then. And then some more when I was still feeding 18 months later... I still have those ones and hope they fit when ds is born and I'm breast feeding again!