for div 3 i would make sure the arena is atleast 8000/9000 seats and go from there.
build more seats NOW (so you can benefit from it a little this season too). and then start accumulating more cash to buy appropriate players which should have salaries above 10k. Building your arena should be done before you buy players with relatively high salary so that you know you will be able to afford them without a big sacrifice to weekly net income.