Honestly, not trying to be mean, I think you will struggle greatly at the higher level. This is for a couple of reasons:
1. Your team is very old... 7 of your best 8 players are over the age of 33, which means that all of their skills are beginning to diminish. I see that you have a couple of young promising players, but you should still look at trying to get an overall younger team IMO.
2. Your arena is tiny. It is not even 8k yet, which isn't a great thing. Personally I would expand it right away, because you aren't going to be very competitive in D.III until your arena is at least around 10k. I wouldn't bet on you getting that many wins this season, but if you could manage to train your younger players and manage to stay out of the 8th spot and stay up in D.III, you could nab a good draftee, and be very competitive in the future.
Overall, congrats on beating out the better team in IV. and being the underdog that succeeded, but you will have a couple of seasons of rebuilding before you can be competitive in D.III. I wouldn't add any more people to your roster unless they are young and you can train them or you get them for very cheap and I would mainly focus on training and expanding your arena for now.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs