Currently, my players are training with an average intensity, so I'm gonna keep it this way.

How much seats should I build in my stadium ?
Is it more important and interesting saving the money to hire a new coach and not increase the capacity of the stadium for now or spend the money building the stadium and wait for the coach ?

it depends on your outlook. If you want to train faster, a coach with one good stat is the way to go.

if you want to make money faster then the stadium is your best bet

I'd really need your advice about the future of this guy :

21 years old

SKILLS
Positioning 4 Tackling 5 Punting 7
Vision 4 Blocking 5 Speed 15
Intelligence 16 Passing 6 Strength 8
Footwork 3 Catching 7 Agility 4
Carrying 4 Kicking 2 Stamina 5
OTHER
Teamwork 19 Consistency 15

His talent is 4 stars.
I dont have a good QB in my team, or the potential QBs are getting very old (more than 30y old) ... That's why I'm looking for my future QB and if it's possible without buy him from the market. Yesterday, watching and watching again my players I stopped on this guy, young, a great talent, an intelligence a teamwork and a consistency very great! Especially the teamwork because I wont hire right now an offensive coach thus he'll be able to improve quickly enough. Actually I was hesitate because of his tackling, indeed I miss currently two good def so if he has to play QB the weakness of my defense increases... He would be a great def too, so I hesitate. What do you think ?

Thanks a lot.