I'd stick to a level 4 or level 5 at most.
In in entire history on BB, Ive only ever had a level 4. Which explains why my trainees never make the U21. I sack the trainer when he gets too expensive and train stamina/FT/gameshape for a few weeks cos you dont need a trainer to train those teams skills. Then I take my time to search for another low salary level 4 trainer.
If you want to train for U21 then level 6 or 7 is the way to go, but I wouldnt reconment it cos they cost too much in salary. But yeah if you have a lot of money and your primary focus is to train then go for it.