For your natural height, it would be very hard for you to comfortably fit into a zero while still maintaining a healthy weight. There's nothing wrong with being thin or wanting to be thin, but you also need to take your health into account. I'm about the same height and quite a bit heavier, but people always comment on how thin I am, even though when I wore women's clothing, I generally wore an 8... I could squeeze into a 6, but I've never liked tight clothing *being FTM I don't like to show off my womanly figure >.<*
As for whether you should buy women's clothing before you transition, only you can answer that. Do you have money for it? Do you mind the possibility of having to buy more later? No one can tell you what you can or can't wear. If you're more comfortable in women's clothing, then please wear them. If you want, there are even fake breast forms that you could wear inside your shirt so that if you end up with a similar size after top surgery, the shirts will still fit.