Oh, the desire to play a bored priest suddenly rising.
To repeat the above: Good Sturdy Wa Shoes are a must. You will be up and about all day, so comfy, hardy shoes are a must. Pack 2 extra pairs of socks to go with.
2nd Remember to eat. I’m terrible for this and end up skipping meals because i’ve been running about.
Costume: Tbh, If you turn up to your first event in very basic kit (plain black trousers, simple hero shirt ect), no one will look twice. You may not stand out as a citizen of X nation immediately, but it will get you started. Then at your next event you now have a pair of trousers that match you brief better and a cool swanky hat (A sure fire way to get into character) all the better. There is a culture for “Aspirational Costume”. You will see full length dresses and hand stitch robes and fancy embroided pieces. No one expects you to have these, or to get these straight away if at all. But we all try and make improvements from event to event and thats all that is really asked for.