I recommend that you dont train any trainees below the potential of all-star. It is better for your team in the long run because of resale value and usefulness to your team if and when you promote. A capped 6th man or starter potential wont help you much in upper divisions so its better to start training a player that can be as good as you need him to be for the foreseeable future.
In the short run, there wont be much difference but in the long run, I guarantee you will be better off either in player value, and/or team finances with training a higher potential trainee. It's just easier than buying better players as you promote, when you can keep training for free essentially.
allstar/perennial allstar trainees dont go for much, especially at this time of the season. If you want to be fancy, then picking up a superstar+ potential is ideal (that is if your finances allow you to do so)
a few guys up there pretty much covered the training/staff portion of your question so im not gonna repeat others.