Well first off unless no one in your league looks to make a run on you in the end of the season, you should go for the title.
If you have the cash, you should look at building your arena once you are sure no one else can out buy you to a playoff championship.
So you should look at dumping in your arena shortly after the playoffs start probably.
I would go for 15 boxes (600 or so price), 300 court (80 or so price), 1000 lower (35-40 price I guess), and 6000 (15 I am guessing) bleachers is probably a decent start.
About your team for Div4: I would worry about building your arena and perhaps attempting to make the playoffs next season.