So I'm assuming if I want a team loaded with "marvelous" players, I train a couple and buy the rest? (Over several seasons.)
yep. 
So I'm trying to narrow the focus of training to 4-5 key players, young with high potential. Like my 5-star draftee! 
Honestly, if you are new-ish to the game, try focusing on 1-2 players. Its really REALLY difficult to train 4-5 players all at once. 
Some training concepts which might help you. 
- Single position training is bar far, the most effective. Running 2-position Pressure/Passing/OD training is pretty much a waste of time. 
- 48 minutes each week in the position to get full training. The difference between 40 mins vs 48 mins is massive, its not a linear scale, its logorithmic. That is, 40 mins is like, 65% of full training (or something like that). 
- Younger trainee's train WAY faster than older trainee's. You will notice a significant drop off at 21, and then again about 25. 
Hope this helps.