In my opinion, competent trainer is enough, if you train some lower potential players. Also if you train 3 players.
I mean you often get 46-47 minutes. From 48+ minutes, it's just like having a basic trainer instead of world renowned.
I would take level 5+ trainers only if I'm building a top notch player. Anything below, it's best to take a competent coach, 3 trainees, and don't ever worry about lost minutes or injuries.