As above
On the subject of arena, max boxes is 50, max courtside is 500, I'd probably these days aim for 13k, 7k, 500, 50
In terms of training, yes Allstar or better, from scratch it's only worth putting major effort into training 19 or under until they are 27/28 (players train faster the younger they are and the better coach you have). For starting advice train defenses, defense is critical to carving out a good season position.