Minutes. Most people would recommend getting 48-75 mins per player per week to maintain/improve their Game Shape. I however take the past month's minutes into account and try play or rest a player so he'd stay within the 60-65 average. But that's just my method.
If you had/have a player we've identified as potential U21/NT, we'd have immediately sent you a BB-Mail before you even know the player's name. We'd have also advise you to get at least a level 4 doctor. But for now you don't have any outstanding trainee so a basic/competent doctor is ok for now...
Don't waste money on expensive players, save them up for arena. Maybe when you promote next season you can get a few... but important thing is arena + draft. Don't worry about your league position, next season, through bot cleanup you will 99% be promoted.