It's very difficult to know how much money you'll need each month, and what expenses will crop up.
But i'll do my best to answer the questions I can:
regarding nappies - I think on average a baby goes through 5-6 nappies per 24 hours. Though newborns can go through as many as 10 as they poop a lot. As the child gets older and moves into bigger nappies, there are fewer per pack if using disposables, which makes the nappies more expensive. A toddler can easily go through 2.5 - 3 packs per month, which would be about R260 easily per month.
A way around that monthly expense would be to use cloth nappies. these have a fairly high start up cost, but if you can save up for a few months before you get pg and whilst pg and get these, then it's one less thing to worry about from month to month. There are loads of different kinds of cloth, it's not only the old terry towelling squares available anymore. Many are just as easy to use as disposables. You can check out
www.doodlebums.co.za for an idea of what's out there.
If you go the disposable route you can easily buy in advance. Just keep the slips in case you need to swap sizes. Most shops are willing to swap if the pack is unopened (e.g. if you have too many size 2's and baby is now already in size 3).
You can also buy wet wipes in advance. Provided the packaging stays unopened these last quite a long time. I had some from 2 years ago that I used and they were fine, you sometimes just have to turn the pack upside down every few months so the liquid distributes evenly (if you're keeping for a long time).
If you buy wet wipes and disposables in advance you can look for specials and promotions and save a lot taht way as well.
Regarding milk - this you will have to wait and see. If you can breastfeed that would be cheapest, but sometimes for whatever reason that is not possible. Then you need to find a formula that agrees with your baby. In the early days it's not too expensive as they don't drink much, but you can easily budget on about R400 per month as they get a bit older.
Baby toiletries - if you have a baby shower you will get a lot there. My son is now 15 months old and I have only bought baby body wash once. Other than that I still have loads of baby shampoo from my baby shower, and have used other toiletries from my baby shower stock.
I wouldn't bother buying vitamins in advance - you never know what your child will need.
I did make almost all my own puree. I find it cheaper, but i also like to know what exactly is in the food.
I just have a very basic stick blender and that worked fine.
I would DEFINITELY make sure the baby is at least on a hospital plan. We landed up with 7 hospital visits in my son's first year. According to our medical aid statement the costs were worth well over R75 000! We didn't pay a cent. I do pay out of pocket for doctor visits and medicines though and that also added up very quickly. So i would advice at the very least a hospital plan, and then open a savings account of sorts that you can put money in monthly for day to day medical costs, and start saving NOW. I think we spent over R10 000 on day to day costs in 6 months. We did have a very bad run of illnesses though, but that does happen quite a lot if you land up having to send your child to creche like I do. A child's immune system only develops fully by 3 years old so they are very often sick with colds and other things in the first year or two if they are in creche.
An education policy is a good idea. Also look at other types of saving policies (alan gray etc) though as some others will give you much better interest and more freedom to use the money as you need it.