Buying a prom dress

I am going to get a prom dress at the weekend, I wish it were for me and I was 18 again but alas no, it’s for my niece – she’s left it until the last minute because she didn’t think she was going and now doesn’t have the time to go and get it for herself. I am quite nervous about getting the ‘right’ thing, she’s given me a list a mile long of requirements!