Wow, congratulations!! That is so exciting!!! I'm due around the same time as you, as well! It's a great time of year to have a baby!!!!
IME, you can have a baby for pretty darn cheap. Our babies feel nearly free, really. We have them at home, we breastfeed exclusively, we cloth diaper (only an initial cost), we co-sleep (no need for cribs and all that stuff), we don't use a lot of the baby "stuff" (baby bathtub, exersaucer, highchair, etc), don't to "baby foods", never use bibs, bottles, bumbos, our health care choices aren't exactly "mainstream" so we don't end up spending a lot of money on that kind of thing, etc.
I am a HUGE breastfeeding advocate. If you have trouble, PLEASE seek out a good LC or LLL leader. They will help you! It's SO worth it. We're extended breastfeeders here (WELL past a year...but I don't care to be judged here so will leave it at that

), and the benefits are incredible.
Cloth diapers are like a hobby once you get started. In the last 3.5 years I have pretty much tried one of everything, at least! LOL I HIGHLY recommend getting different types to try instead of buying a whole system of one type to start with. A great benefit to CD's is that you can buy/sell used. Places like Diaperswappers.com are great for that. I have used gDiapers (though only with cloth inserts and not with flushables) and they worked okay, but I probably wouldn't recommend them. Though they're new style diaper is MUCH better than the original. I HATE one-size diapers, particularly BumGenius. They are marketed to be much better than they really are. Granted, the BG's have come a LONG way from their 1.0 to their 3.0's, but they still need to make some serious improvements (mainly in their velcro tab system) if they want their diapers to hold up well for as long as they market them for. I really prefer sized diapers, though. Better fit, for sure.
As for a carseat I recommend the Graco Safeseat (or now I guess it's the Snugride 32). Chicco has a Keyfit 30 but that extra 2lbs and 2" can make a big difference! Other infant seats will likely not last your child the first year...some babies are out of them before 6 months! With the 32, though, there's a good chance it will last you LONGER than a year if you choose to continue using it then. THEN you can find a good convertible. (I agree with the albeebaby.com rec. for buying seats) Convertible carseats are often designed to be used from birth but really, if you see a teeny tiny newborn in those, it's obvious they're not made FOR newborns like infant carseats are.
I HAVE purchased a few BIG spendy baby items, the most expensive being my Phil & Ted's double stroller, but even that I got used. At first you might have to be the one to buy the baby things you need, but from then on you can consign them all if you need to get rid of it and get something new. Same with clothes. Saves a ton of money!
I also highly recommend baby carriers and slings. Having the baby close to you, especially early on, will really benefit him/her. I love pouch slings and wraps for early on (hate ring slings!), and SSC's for when they're older, but you really just have to try them out and see what works for you. ITA though with avoiding the ones you can find just about anywhere, like Infantino, Snugli, Bjorn... Some good ones are Moby Wraps, Hotsling/Peanutshell/Karma Baby/etc pouches, Beco/Ergo carriers, etc. They will also be a bit more expensive BUT you will TOTALLY feel the difference, and you can usually buy them used.
Baby stuff is a HUGE money maker and they know it. Half the stuff out there is SO far from necessary! My advice would be to not feel like you have to have it all right from the start, but wait and see what you want/need.