Lots of opinions here:
Charity shops will become your friends. Look for cheap rings, bits of bling, you might get lucky with some appropriate clothes too.
Masks can be bought relatively cheap from eBay or similar. I even sell/rent some IC.
Depending on weather you might not need more than a base layer and a flouncy shirt as a male. Cloaks can be made cheaply (wool rectangle and a brooch) or bought onsite for not extortionate prices.
Backgrounds are more sort of “guidelines” and league backgrounds can have a bit of overlap. Mountebanks are great (bias here) but a lot of league players like to draw on the cheeky chappy image. I personally see them as good indicators of what is happening in a nation and the possibilities of game available.
General RP advice: follow the improv role of “Yes And…” - be prepared to run with an idea and add to it. Don’t be afraid to be the villain in someone else’s story. Everyone at anvil is a hero but some play more of an anti hero role. Some even play less savoury sorts. Check in with people out of character if you feel uncomfortable or are worried they might be uncomfortable. Everyone’s here to have fun, be excellent to each other.
And finally be friendly to people before time in (6pm on the Friday) and arriving as early as possible (can arrive even on the Thursday) can help you relax and meet people out of character before the IC political intrigue starts up. You win brownie points if you help people set up their tents or even help with the big ones (hospital/theatre/bar tents)