all-star is minimium, because you could probably succesfully 1 position train an allstar for 3 and a half-4 seasons. With a star it takes you less, however, i think anyone could pull of a SF star for an U21 team to come off the bench:) However, i agree with the "dolts" ( :p ). Do not train anything under 7, because it's useless. Ofc, you might get lucky and sell a guy with potential 4 with 700k and buy something useless(seen it happen) , but this is very rare. A guy with potential at least 7 can go a very long way. Longer than you would expect.
For instance, if you train a guy with potential 8-9, for about 6 season(let's say PG), he would be a 150k PG at 22-23, that will own, any league he is in.