Well, the advice I was given was to purchase at least one while in the late weeks of pregnancy, so I bought one. And then my milk came in, I was engorged, still had lochia, exhausted, and was in no state to go to the store. I specifically remember two horrible things about this- my INLAWS volunteering to shop for some nursing bras for me (um NO), and my husband saying he could shop for me. When I asked him how he'd know what to buy, he held out his hands, cupping invisible breasts, and said, "I'll just tell the salesperson that you're about this big, okay?" HA!
So I ended up waddling in the store to buy some more nursing bras a day or two after delivery, so then I guess I had three. And I was leaking through those so fast that I couldn't keep them washed fast enough, even with an assortment of nursing pads. So I'd recommend you buy at least three now that are pretty comfy and stretchy- maybe those marketed as nursing sleep bras- and then being fitted after the birth and getting the nicer, more expensive ones at that time.
As far as size goes, I went from 36b prepregnancy to 36c while pregnant, then 38dd after giving birth and the engorgement had gone down a bit. By 6 months postpartum I was closer to 36c again, and when I was nursing my daughter at 2 years old, I was back down to almost a 36a. My point being, depending on how long you end up nursing, whatever you buy now is likely to fit again at some point. So go for it, but don't be fancy.