Guess what: This is an expression used to introduce a surprising or exciting piece of information. In this context, Mary is using it to announce something interesting to Rachel.
nomber jackets: These are short, zip-front jackets that were originally designed for military pilots. In the context of the dialogue, Bill mentions that his dad used to wear leather bomber jackets when he was younger.
load of stuff: This phrase generally refers to a large quantity of items or belongings. In this context, Bill is mentioning that his uncle lent him a significant amount of clothing and accessories for a fancy-dress party.
rels: This is likely a shortened or informal way of saying "relatives." In this context, Mary suggests asking their parents and "rels" for help regarding the retro-themed party.
Brill: An informal abbreviation for "brilliant" or "great." Bill uses it to express enthusiasm or agreement with the idea of looking for retro clothing online.
sunnies: This is a colloquial term for sunglasses. Bill mentions that he looks great in a black leather jacket and sunglasses when discussing the retro style.