Of-course all, because they all affect your dwellers journey. BUT if you want to find more weapons/outfits/caps which are most important at start you need to focus on Luck/Endurance.
"The level seems to affect how often stims are used." - Level affects HP but not how often stims are used , if you have good Endurance you will survive longer because you will receive less damage, if you have low strength and/or low weapon your dwellers won't fight and you will receive more damage. Luck affects how often you can get caps/weapons/outfits and maybe common/rare/legendary. Yesterday my dweller with 10 luck in 2H and some minutes found a rare weapon 15+ damage, in the evening i will post image.
Well this is at least what i feel and see.