This is a general statement that shouldn't require the personal pronoun. "לא צריכים תואר..." should be accepted.
מה הבדל בין כדי ו בכדי?
I'm not sure there's a difference apart from בכדי being outdated.
There is no difference, בכדי isn't used out of formal context
I was taught כדי (rather than בכדי) years ago. If it's right but just a bit outdated, it probably should be marked as correct, no?
כדי shows very high usage score, while בשביל is much lower. For native speakers, which one sounds more usual?