For the bras I waited until baby was born since I didn't know what size I'd be. However once he was born I found I was too overwhelmed to deal with measuring, sizing, and then finding and buying something so I was in frumpy triangle sports bras for awhile. Everyone's different though so you may find you have time/energy to deal with finding the right bra. If I did it again I would still wait, but I'd find some stores and styles I want ahead of time so all that's left is to measure and purchase. As for nursing clothes my advice would be not to spend too much on items specifically for nursing as they can be a lot pricier than normal clothes. Things like button/snap down shirts, criss-crossed v-necks, basically anything with easy access can work well and then you can still wear them after you're done nursing :)
The nursing tank tops from Walmart were great I had my son June of 22 and be totally honest . I went shirtless and braless around the house most the summer last year with the heat and convenience it’s what worked best lol !