It depends on the level of your Trainer, as well as trainee height and what skills you train over time. For example, the elastic effect helping increase the level of a much lower skill.
When it comes to TSP, I stay away from guards with bad OD/PA and bigs with atrocious OD/HA. For 19 year olds, I'll be more picky and won't pick a player with glaring holes in the skills I just mentioned. Furthermore, I avoid tall guards and short bigs.
I don't like to spend too much, because I can typically find a decent trainee for a cheap price.