1a) Most people wear some combination of a t shirt and jeans or chinos, you might see polo shirts, button downs or joggers every now and again then in the winter months hoodies, sweatshirts and the occasional letterman jacket.
1b) no one cares
2) Just pick a button down and you're good, no one cares about brands, just buy what you want
3) People care even less about jeans
4) Knock on doors, ideally with a case of beer/cider to share, drink in your flat for a bit then head out for food/drink/shops, etc.
5) It varies but I'd say people with big rooms tend to keep their stuff in their room and those with smaller rooms keep stuff in the kitchen.