For 18 years old trainees, I look for 50-54 TSP at the start of the season. For 19 years old, 60-63.
If training centers, I look for 8 potential upwards, but I still prefer better starting skills over potential.
At the beginning of the season, they are not difficult to find, they are not as expensive as the top trainees and you can still build nice players out of them.