I think 21week is okay, not early at all. Cos by the time you reach 7 or 8mth preg, your tummy will get bigger and is very tiring for you to shop and source for baby stuffs. I also bought lotsa baby stuuf once I know the gender and 2 mths before my edd, I alrdy bought 90% of the stuffs.
Diapers dont get too many packs and get get last min since most of the places sell diaper. Milk bottle, breast milk container, steriliser, clothes, mittens, booties, hat, cot/playpen, pram, wet wipes (can get at later stage too), breast pump, nursing bras, laundry detergent, bottle detergent, bean bag etc etc!! You see, so many things to buy, so better start now haha! Anw, for certain items like cot, clothes etc, you can get pass down ones from friends/relatives if you dont mind. =)