My opinion is that they need more OD. Training them will take a lot of time. If I am looking to progress faster, I would sell them. Considering arena, I believe that arena should be big enough to play at one division higher. So that you can pay salaries for better players. Your arena is good enough for D3. I would probably buy new players to train or older to improve your team and try to expand arena in some future for competing in D2.