Congratulations on the baby!
I too have struggled with depression and anxiety, and after 6 years of trying all sorts of medications and therapy, I decided that approach was not for me. I agree with your idea of committing to a "healthy lifestyle - plenty of exercise, good food, enough sleep, and a fairly regular schedule"...for me, that made a huge difference. But when you have the baby, those things are difficult to accomplish. And as you said, you will have to go through the adjustment of having another family member and big time sleep deprivation. Those things, and your hormonal changes cannot be controlled. You may indeed need medication even if it is just for a few months.
But, there are some other things you can do to help make it easier
plenty of exercise: if you can afford it, a membership to the gym would give you a place to be away from the baby for an hour and get your exercise in, a shower, and a way to socialize with adults a little. Otherwise you can enlist the help of a low-cost sitter or family members or join a "mommy and me" exercise class
good food: before you have the baby, make a bunch of healthy meals and freeze them so they are ready to be heated up. Have your family set up a schedule where they will deliver you a healthy meal once or twice a week, or go grocery shopping for you so that you constantly have plenty of fresh stuff around to snack on
enough sleep: this one is tough. You can have people come over to babysit and let you take a nap, but no matter what, you will still be getting up all night long. Everyone told me before I had my son "whenever the baby is sleeping, you should be too"..it IS great advice, but it's hard to do. And with another child to take care of, it likely wont happen very often. The only thing I can suggest is to make sure your husband, family, and friends know how important it is and to help you whenever they can. You might also want to try a drop-in daycare occasionally for your son. Over time you will be able to get more sleep. Remind yourself, so that you can see "a light at the end of the tunnel"
fairly regular schedule: When the baby is first born this is also tough. You are on baby's schedule after all. But when the baby settles in, it is easier to know when the baby will usually sleep, when he/she will usually want to eat, etc. The baby will basically set your schedule for your for now, but you should plan special time alone with your son, and family activities like going for walks at the same times every week and eating meals at the same times every day. Eventually your baby will get used to the family schedule
You have probably heard/read those suggestions already but I wanted to write them anyway.
Even if you decide against them, three things you definitely should do are 1. Keep coming on here! or join a chat room for anonymous support 2. Join a mom's group/class for something to get you out of the house, as well as for support 3. Forgive yourself if you did not "perform" as well as you thought you should have