I really stressed about this & went overboard on the clothes dept. I go by the basic rule that you put an extra layer on so for me that would mean a singlet under his clothes.
For me I hated dressing DS too often & all those cute outfits... are just that, cute - but a real pain in the a$$ to dress him so he lived in the all-in-one suits, that button all the way up the front or zip up so I didn't have to be putting clothes over his head etc... I have only just started to put him in pj's & trackies (so cute)
Sept is a strange month (a bit like march except it gets hotter instead of cooler) some of the suits are lightweight while others are thicker material, have 1/2 thicker ones but you won't really need them cause while they sleep you swaddle them & put a blanket on them anyway.
Also TRUST yourself, instinct does kick in (& yes I didn't believe that either when I was told "you just know" like how? )