DS was 8lbs 4oz at birth (3.740 kgs) so he fit into 0000 for quite awhile but it was also summer so he wasn't wearing long pants or long sleeves. I would only buy a few items in 0000 as you can always buy more if you need them.

DS wore size 0 for the longest - started wearing size 0 pretty consistently from 6 months and only at about 13/14 months did he start wearing size 1. He is now 15 months old and still wearing size 0 jeans and long pants - but size 1 all in ones and shirts.

Sizing at different stores and also different brands ranges as well. I find Bonds runs very small - Kmart clothes also tend to run small. Pumpkin Patch and Big W are average and Target runs large/long. Therefore I try to get a majority of DS's winter clothes from Target as I know they will last for awhile.

I hope that was the kind of information you were looking for and I hope that helps.

Oh, my other piece of advice is amount of clothing... DS was a poo-er (every feed) and a spewer and I often did a full wardrobe change after every feed. I didn't want to be chained to the house or the washing machine so I bought lots of clothes instead. So that is another thing to think about.