I think nursing tops are probably handy, but don't get too stressed about them - they're not essential. I haven't gotten them for nursing my babies so far. Although, I have been tempted - they do look convenient! But the price is always too much in my mind.
I did get myself some belly bands - they sell them on e-bay for about $10/piece. It's a stretchy band of material that goes over your belly and keeps you covered when you lift your shirt to nurse. It's also great for helping maternity pants stay up, or for covering the waist band of your pants when you are in the transition from regular pants to maternity pants, or vice versa.
If you do decide to get some nuring shirts, maybe wait until after you have your baby and your milk comes in. Some women find that their bust size changes drastically and you might be unable to wear the shirt you bought before.
Otherwise, just get a couple of comfortable, loose-fitting and LONG shirts, and they should do just fine for you to start with anyways.
All the best!