The best value for money is still a level 4 trainer.
Level 6 and 7 are a waste of money.
Not many teams starting out can afford level 5 trainers. The difference between 4 and 5 is an extra 2-3 weeks worth of training a season so you're looking at an extra pop or 2 in skills. As demonstrated in other threads, 4 is still the best unless you have a massive training regime and all your players for that position are trainees