You don't need anything at all. That's one of the many good things about breastfeeding - it's free!
Some women do 'leak' a bit in the early days, but don't buy too many breastpads. Any leaking will sort itself out quite quickly as your supply adjusts to the needs of your baby.
You shouldn't experience any pain or discomfort while you are breastfeeding - if for any reason you do, get your midwife or a breastfeeding supporter to help you as quickly as possible. Getting the baby attached well to your breast is the most important thing, and will prevent most problems. I know that lots of women swear by Lansinoh, but if you do get a bit sore in the early days, vaseline is just as good, and keeping your nipples wet with breastmilk helps enormously (breastmilk has antibacterial properties). Your nipples are make from the same stuff as your lips, so make sure they don't dry out.
A couple of comfortable nursing bras can make life easier, but any stretchy bra will do.
If you want to feed when you are out and about (and you probably will, unless you fancy being completely housebound), it might be worth thinking about what you will wear if you want to be discreet. You might not need to buy anything new, and you certainly don't need to waste money on 'proper' nursing tops. Two layers of stretchy vests are useful - pull one up to cover the top of your chest and one down to expose just enough breast for the baby. Big shirts are good too; you can simply put the baby underneath.
I think the best investment you could possibly make is to know as much as you can about breastfeeding before you start. Are there any breastfeeding groups in your area? The book The Food of Love by Kate Evans is a good read and has some lovely pictures.
Good luck 