I bought a few comfy bras a size up since I figured when my milk came in my boobs would get bigger, so I'd need to buy nursing bras in a different size then. I think if you're boobs get larger during preg, you can estimate another 1/2-1 cup larger when milk comes in. At least that's what most women tell me and what happened to me. Now if your boobs don't get much larger, you may get away with just buying nursing bras. Remeber though, that what is comfy now, may not be when nursing. I could wear a wire during preg last time, but soon as ds was born I switched to soft cup-1 so I didn't run into duct issues 2 they were so much more comfortable with my boobs constantly changing size with engorgement etc.
This time I'm on the search for nicer bras, regardless of price. We're ebf at 19m and still going now, and I plan the same for the next one. I hated the bras I had with ds. My boobs dropped down around 12m so I'm back wearing my regulars now, but already they seem a little full. I'm searching for lightly padded, soft cup bras this time, so as not to show the pad etc. It seemed no matter what I wore, you could always see my nursing pad or nips and I hated that. I did take the wire out of my one motherhood one and it still worked, but that was the only wire one I had. Oh, I'm a b/c normally, went to a d during preg somewhere around 3m, and then a huge d after birth. Clara I prob didn't help you at all did I?
My boobs have gotten a little bigger - I just about fit now... 
