If your arena is still small, I think the money spent building it up would be well worth the investment.
In the last half of my Div IV run (I went 21-1, won the playoffs...), I was able to squeeze an extra 2,000 seats into my arena which has really been of benefit this season. With such a great record, your season ticket holders will be up (mine jumped by close to 800, to 1296) and you will fill those seats. And a strong Div IV team is probably equivalent to a mid level Div III team, as in my case. I've been virtually able to sell my arena out all season and have made significant financial gains this season. To succeed, you will need money and a consistent source is from attendance, IMO.
The trainers and players need the foundation of income.
I'd say, expand that arena to at least 7 or 8 thousand. I am working my way from 7 to 8 at this time.