Ditto PPs- it's very hard to know what size and brand you're going to need until LO gets here. Some babies are allergic to certain brands or you'll find some are just "leaky" on your kid- you'll need to wait and see.
Also, diapers get funky if they sit around too long. The get kind of stiff and don't absorb as well, so you don't want to buy a ton of diapers now that are going to sit around for a year and leak when you use them.
I would try to just save money for now and as you see great sales on gear, furniture, etc. buy that stuff early on. Closer to you EDD, pick up one or two small packs of NB and size 1 diapers. Most hospitals will also send you home with a few days worth of diapers- if you like what you have, order some more on diapers.com and you'll have them on your doorstep in a day or two, if not, try something else until you find what works. And don't stock up too much in the smaller sizes (<3), if they go through a growth spurt, you'll be left with a bunch of too small diapers.