Wow. You are in a truly remarkable situation! You will have to spend literally nothing on your arena for the rest of your BB career.
Right now the worst thing you can do is sit with this much money in the bank and transfer prices essentially bottomed out. There are many different things you can do here. Since you're on track to turn a $150K profit, you can add some big-time players on the transfer market and compete right away.
If I were you I would run -- don't walk -- to the transfer list where you can snatch up a couple of $40-60K bigs for a million or less each. Explore the possibility of using the rest to get a superior trainer. I prefer single-position training, but in your case you have a great setup for double-position training. You have five guards -- Betts, Fultz, Barzanti, Pashazade (listed as SF), San Roman -- who are between 20-22 and are 6'3 or shorter. No, they aren't perfect trainees, but all will still improve pretty quickly if each gets 48 minutes.
You are in a great position both to compete now and to train for the future. At the end of the season, you'll be able to sell off a couple of the trainees at a nice profit and fill in any other holes you may have. Don't be afraid to be aggressive here.