for the first months, i think it depends on whether u intend to breastfeed baby or not (nothing much to take other than small diaper sac and blanket) and how far you intend to go out with baby (usually not very far...) i got a special "mother bag" but ended up using my normal bags most of the time. btw, whether you can easily attach it to the stroller is more important.
i suggest you go around to have a look, just an idea (eugene group, bumbs to babes etc.) wait till baby's born, decide whether u breastfeed, go out with him/her 2-3 times, before you can decide what u need and whether u need to buy one at all. i did some research in my first pregnancy and come to the conclusion: there is just little or no magic in 媽咪袋, you will certainly need a bag, more convenient if there are some compartments, but unless you really like the style or material or brand (like any handbag you would buy), don't buy a very expansive one just because you think its really necessary!