D.,
First off Congrads! WOW is your life going to change drastically and there is nothing that anyone can say to prepare you for your change and your little bundle of joy!
Maternity clothes:
For my first born, I never really had to purchase clothes. I did not really show until I was 8 months pregnant. By then I just purchased a few things from the local maternity store. Shoes, I wore whatever felt best. My feet would swell so bad when I would get home from work, that most of the time I just took my slippers to work with me and a comfy pair of slip on hoes for getting in and out of the car. Bras... WOW! I went through a ton of them cuz my breast went from a size 34C to a 38DD, so I had a continually compensate for that.
For my second child, I began showing before I even knew I was pregnant. I would be flat as a rail in the morning and then at night I would be all bloated up. I needed to begin wearing maternity clothes when I was 4 months pregnant with him. I used some of the clothes I purchased with my first.... but I also purchased "lots" of clothes off EBay. I found that I needed a size medium with him where with my first I was in a small and Xsmall.
For my last, I began showing immediately. But.... I only pulled out a few of the maternity clothes (I am now trying to sell them on EBay!) One thing that I found with her is that I needed a size large. With her I broke out in a horrible rash on my belly so the normal maternity clothes really bothered it as they cover the belly. I also found the belly cover in pants and shorts to be very uncomfortable for you unless your belly is really sticking out there. So, I just went to Kohl's and bought a ton of XLarge sweatpants. They have some that with the right top could be considered nice work clothes! That was much more comfortable for me and I still wear them to this day. I just roll them a bit. Bras.... I decided to get sports bra's with her. They were much more comfortable than Bra's and I did not have to worry about the sizes and getting news ones. Plus I felt supported better. Shoes.... I was actually just barefoot with her and then a nice pair of slippers and slip on shoes.
Good Luck! and again..... congrads!