Hey and welcome ....

Sooner or later the system will force you to upgrade the training facility to proceed with other facilities.
I don't advice you to buy the Academy players unless they are above 80% in key position such as DC-MC-ST... you can get better than them from auction if you are patient ...
A balanced defense is made of 4 players with 2 defense centers and two half backs and a secure defense is with an additional DMC.
I used to sell everybody and buy new players every season at low levels. But If go back in time, I'd not have done it on wide scale. Just keep the best young 4-5 and train them hard to gain 100% back.
Attributes (white ones) are important ... the higher they are the better.

Good Luck