Just a few things to add
Re sizes - Ethan is the only baby I know who hasn't gone straight into 0-3 month clothes and bypassed everything in newborn. My family never knew you could get prem clothes before he was born. If you were unlucky enough to have a prem / teeny tiny baby you can get stuff pretty quickly (in that case are you going to buy prem nappies etc too?)
Clothes fitting - sorry, but that won't happen. At the moment Ethan (8 months) fits BHS prem clothes right through to M&Co 12-18 months. Unless you want to try on every item of clothing every day of their life, you will end up putting your baby in slightly baggy clothes.
Colours - if you want basics, I would always say get some white. Bright vests are all well and good, but you spend all your time making sure you can't see the pattern through their clothes
Price - when it comes to cost, spend what ever you want, it's your money after all. But please bear in mind that you will often bin clothes before you have even done the poppers up in the early days (poo really travels) - I often felt I may as well just stick the money in the nappy and use it as a liner for the amount we wasted.