I would suggest since you are likely not to make the playoffs to start spending your cash on the arena.
However, I'd also suggest getting some more players that can win you games but stay close to the minimum salary for this season. Try to win some so you get more people attending your games.
I would concentrate on building up to 15 or so luxury boxes, up to 300 courtside, 1000 lower, and 7500 bleachers. Since pricing is about to take effect until the end of the season, you may want to look at buying a few decent players while staying near the players salary floor and then expanding before next season.
Your priority for next season should be to win every game. So you probably want to be assured you have the roster to do it early next season.