Just for reference, a person trying to create a NT or U21 level player will go for a level 4 or 5 trainer. So I agree that lvl 3-4 would suffice for where you're at now.
And I'll just second that it's probably worth replacing the second trainee you mentioned, due to his height and low starting skills. You can find better prospects for cheap right now on the TL. I suggest you find one.
Beyond that, I created a Training Simulator
(229484.1) with data from the BB-Training Project
(203921.1) which may be helpful for you in experimenting with what to train. And the Training project will help you see how much you get out of each training.