1 position training for new teams is not the most wise advice that u can give. For sure training faster but much less players. I am playing since season 6, and maybe i will try 1 position training after this season. Why couz producing players=money to built u r team.
Training 1 position is luxury that i have now that i am top2 team in my country.
About the trainer considering how much expensive are 6-7 lvl, the most wise choice is lvl 4. Cheap to buy him, low salary. Even if u want to upgrade go to lvl 5. not higher. It just doesnt worth it considering how much u have to pay to buy 1+salary