I would say as much as you can. The key is to balance having a competitive team, training young players and having a decent sized arena. You can't compromise either one of these and you would be alright. I would probably aim for winning the IV league and getting to III, where the rewards are much higher. Then building your arena to 10000. I have a friend who has a 15000 sized arena and his weekly income is 168k and he is in the middle of the league strength wise. In IV league i would go for 8000 maybe?