We had planned on a year or two before starting a family, but about 5 months after the honeymoon, I missed my period and our whole life changed.
In hindsight, I wouldn't have wanted it any other way. As women, we find having children fulfilling in so many ways, and I think we can get too selfish if we focus on the "just us" stage for too long. You're being smart to try to lose weight for the pregnancy--I am thankful I was my ideal when I got pregnant (for the next one I probably won't be quite as ideal, but I'm still working on it!).
Financially, it can be as big a change as you want it to be, I think. We couldn't afford to buy a crib an changing table, etc, but my mom bought those things, so it worked out. Probably you can afford them--but if finances are a big issue, don't buy the fanciest; you can find good quality used furniture if you look; just make sure it's good quality. Mine looked practically brand new--just a few cosmetic issues that didn't affect safety. We were given a lot of baby clothes and so we didn't have to spend a lot on the baby. I also sew, so I was able to make some things to save money.
If you spend a lot on clothes, I would suggest simplifying your wardrobe; try to wear more classics and less trendy stuff, so you can wear it for longer. Then dedicate the savings in your clothing budget to your baby, who will grow out of things LONG before they get worn out.
Why do you have to keep your job? If you look at the expenses your job incurs (gas, eat-out meals, nicer clothes, perhaps the extra car and insurance, paying taxes on that income) verses your total income, you might discover that you actually increase the overall income by very little. If you start fixing your husband lunches, you can probably feed him for under $2 a day if you send meals made from scratch (if you like Mexican food, send beans, rice, and tortillas, for example, and the meal will cost less than $1.50). Cook your own dry beans in bulk and freeze leftovers to save a ton of money! Don't buy the expensive microwave meals: buy the raw ingredients and make your own. It will taste better and be healthier, too. When the days are hot and the nights cool, open the windows at night and close them in the morning, and don't start the A/C until it gets too warm in the house (which at certain times of the year may not even happen). Plan a menu and buy only what you need for your menu. Stay away from junk food that costs a lot but doesn't really fill you up. You may feel full, but your body will crave nutrients and you'll be hungry sooner.
You might wonder what to do with yourself if you stay home. Try picking up a craft that you enjoy (or write a book or learn to fix small engines or whatever piques your interest) and after you've got all the housework done, spend some time with your hobby. I sew, and I actually make some decent spending money with my hobby now and again doing mending.
Since you're going to have a baby, you can save a TON on diapers if you invest in cloth diapers. You can get the fancy ones that look like Pampers (about a $300 investment, but that's less than you'll spend in a year on the cheapest disposables available, and they'll probably last you all the way through to potty training). Or you can get the prefold kind. Gerber sells them. I use them mostly, but I have made my own fancier kind too. Dry them on the line and they will whiten better and it saves money when you don't have to dry them in the dryer.
I wish I had been married at 23 to start having kids. I was 25 when my first was born. I'm now and itching to try for #2. :) So my advice is don't wait. As soon as hubby is ready, go for it! :)