I would thoroughly recommend buying your nappy bag on Ebay. The have lots of brand new ones there that are cheaper than in the stores.
Oioi ones start at about $90, so they aren't cheap, but they are great quality and are really trendy. You can get them from some maternity shops, but I got mine on Ebay. They are also on line, doa oioi search and you should find the site.
I think the best thing about a nappy bag is making sure it can be zipped up, cause at first you have pads, and breastpads, andeverything else in there, and its kinda embarrassing to have everything spilling out. Then with a toddler a zip stops them pulling EVERYTHING out!