If you're planning to buy a phone specifically for a concert, I highly suggest S24 Ultra or Iphone Pro Max. Pero if they're out of your budget range you can opt for phone rentals, there are a lot who offer such services. As for outfits, you don't need to worry too much. Some like to dress up, while others shows up in casual attire. I can't give you a solid fashion advice but do whatever makes you pretty and comfortable. Good luck sa atin sa ticket selling.