Hey...i think i have a much better team than anyone else in my league (IV), now does it pay off to buy some more players to make my team extra good so i can make a run in the national cup???
Also... my arena is bigger than most of the teams in III league
How far you go in the cup will be largely dependent on your draw so I wouldn't add players specifically with that goal in mind. As long as you play D4 and D5 teams, you'll be in good shape but as soon as you play a D2 or D1 team you'll immediately need a hope and a prayer.
I would concentrate on saving your $$$ and trying to promote. And if you are going to promote you should be comparing your arena to the higher division, not your current division.