I think there are two ways to train:
- If you want to win, you have to add trainees slowly since you don't want to have a hole somewhere each game
- If you don't want to win, you can buy three trainees at once and focus solely on them
First option is a bit complicated and that's the route I took; adding a trainee every 2-3 seasons and promoting to D.I along the way. The problem with this approach is you have to take some compromises with training. Not the minutes of course, but the training order.
Second option: With experience I have after 10 seasons of playing, I'd pick up 2nd option instantly and go for three SFs with different heights and superb skillsets. It's hard to pick up such talents at once, so buying all three could span over a season, but not more than that. When they are finished, I'd add a PG and C with few touches of my own training and go for the top of the world.
Also one of the problems with buying trainees every 2-3 seasons is they get very few playing time together as finished products. My PF Navršnik is 24 and he will be 31 or even 32 when my 18y old HoF completes the training.