Within reason, babies can be as expensive or as cheap as you want them to be. Take a walk through Babies R Us, and you'll see that most of the stuff they sell there is unnecessary.
Car seat, crib, changing table, rocking chair, stroller, etc. -- All these big purchases can be found at consignment stores or even yard sales for pennies on the dollar. Since you'll use these things for probably 2 years per child, it makes sense to purchase the items you really want, even if you have to buy them new. Several warnings about these major purchases:
Car seat that've been in wrecks can have hidden damage; I wouldn't buy a car seat from a stranger. Do get a "basket style" infant seat, then a toddler seat later -- infants are just too small for those "convertible" seats.
You probably want to start out with a full-sized stroller that fully reclines; this will be a necessity before the baby can sit up and hold its head up. Make sure you choose a stroller with plenty of storage underneath. Once the child is about 6 months old, you'll probably want a small umbrella or mid-way folding stroller as well. You'll find out quickly that the full-sized stroller is so big and cumbersome that you will only use it for extended outings; you'll want the small stroller for quick trips.
High chair -- You'll be using it three times a day for about three years, so buy the best high chair you can find. Consider a chair that can be pushed up to the table so the baby can eat with the rest of the family.
A changing table isn't a necessity, but it's very handy. I had a $20 yard sale changing table, and it was well worth that price. It's not worth a couple hundred at a store.
The things listed above, however, are not make-or-break items. No matter whether you choose used or new items, you will probably only buy them once. What will kill your budget is the consumable items:
Clothing. This is where you can really over-spend. Since your baby is due in September, I'd suggest that for everyday use you buy a package of onesies and a package of terrycloth sleepers in each size. And tiny socks. Those re the "baby standards". You'll receive many cute little outfits as shower gifts -- many of them probably won't even be worn. Take a stroll through a consignment store; you'll find all the expensive name brands, but they'll be 10-20% of their original cost!
Diapers. Try all the different brands to see which ones you like. Toys R Us and
Walmart brands are every bit as good as the name-brands, but they cost significantly less (even with coupons). I also liked Luvs. Don't over spend on something that'll go into the trash in a few hours.
Wipes are a different story. You can get by with cheap-o wipes for the first few months when the baby is exclusively milk-fed. Once you introduce solids, plan to buy the better wipes. They're thicker, which means you can use just one.
Going back to diapers for a moment: When my second daughter was born, I used to buy diapers from a lady who lived near me (she had 2 sets of twins, all boys!), and she'd found a source for second-quality diapers. She even delivered them to my house for less than Walmart diapers! I don't know where she found them though.
Food. I can't say enough about breastfeeding. I would have done it just for the savings, I would have done it just for the health benefits, I would have done it just for the convenience. Having all three together . . . well, few things in life are so perfect. Having said that, breastfeeding isn't always easy in the first few weeks. You'll be recovering physically, you'll be without sleep, and you'll be uncertain of everything. It's easy to give up. Benefits of breastfeeding: Nothing to buy, nothing to mix, nothing to wash, nothing to transport or warm up. Breastmilk doesn't stain clothing (formula does -- especially ready-to-feed or concentrate types). In general, breastfed babies spit up less, get sick less, and are less likely to have colic. Yes, it means that all the feeding tasks fall to you, but it's only for a short time -- consider it an enforced rest break for you. Once you and the baby have learned how to breastfeed, you don't even have to be awake!
With my second daughter, I had a pillow that was sort of C-shaped; oh, I loved that thing! It helps hold the baby up to just the right height for nursing, and it made life so much easier. With my first child, I always used a bed pillow, but this nursing pillow was so much nicer! I give those pillows to everyone now for shower gifts.
If you need formula, ask at each doctor visit if they have any samples to give away. I breastfed but did use an occasional bottle when the babies went to grandma's house, etc. Because the doctor gave me 2-4 bottles of concentrate every time I went for a well visit, I literally never bought formula (this certainly wouldn't have been enough for a formula-fed baby, but it was more than enough for the occasional supplemental bottle).
When you introduce solid foods, try to minimize your use of those small jars. They're very expensive! Invest now in a small food processor ($40-50). I bought no-salt green beans, carrots, etc. and ran them through the food processor; then I froze them in ice cube trays and saved the "cubes" in ziplocks. I could pull out one cube of pinto beans and one cube of carrots, and warm them in the microwave -- easy and cheap! I always kept a few jars on hand for taking to my mom's house or a restaurant. And babies do need baby cereal; there's really no adult alternative. These hints save TONS of money!
Toys -- Choose a few simple toys; babies don't need a whole toybox full. A few stuffed animals and a few chew-able plastic toys are plenty. The electronic toys are not all they're cracked up to be. Don't skimp on heavy board books though (and I wouldn't buy them used since they really can't be cleaned, and you know they've been chewed).
I think the biggest thing you can do to save with a baby is NOT buy stuff, stuff, and more stuff that you don't need. You don't need an infant feeding chair, a bouncy chair, an exersaucer, an electric swing, and a car seat. You don't need a thing to keep your wipes warm, a cooler for night time bottles, or 100 baby outfits. If you buy with common sense, you can provide your child with everything he needs now AND start his college fund.