Sorry to hear you're suffering from PPD. I'll try to answer your questions as much as I can, but I suggest that you get in touch with your midwife/OB/PCP about this asap.
1. The medication is not addictive in the traditional sense. However, it changes your body chemistry because it addresses a chemical imbalance in your brain. This means, a) it'll take some time for your body to get used to it and for the medication to be fully effective and b) you have to wean your body of the medication and you can't just stop cold turkey because that causes some unpleasant side effects.
2. Many antidepressants are available as generic drugs, making it less expensive to take them. Always ask you doctor if there's a generic version of the med and if so to prescribe that instead of the brand name. Most insurances also cover antidepressants - I believe mental health riders have become illegal.
3. Zoloft is safe for breastfeeding mothers and their children because it does not secret into the breastmilk. This is not true for all SSRI's, though.
4. Most people don't take SSRI's forever but get off them when they've felt better for a while. Often it is a good idea to combine SSRI's and therapy because it allows you to not only feel better but to work through some of the underlying psychological issues causing the depression.
5. SSRIs work by changing the way your brain processes a certain chemical, serotonin. Serotonin is linked to depression. If you google it, you'll find more detailed info.
6. Your doctor will most likely want to see you a few times as you begin taking SSRIs to make sure that you're tolerating the medication well and that you're at the most effective dose. Then, they will probably want to see you once in a while (every 3-6 months) to make sure that things continue to be ok. Your doctor may also want to supervise weaning the SSRI.
I hope this answers your questions. Good luck.