Keep your 18 and 19 year old bigs. Train them single position rotating through Inside Defense, Reb, and Inside shot. For now Keep your players with above 3500 salaries while firing the rest unless they can sell for more then 20,000.
Sell your 21 yr old MVP potential player (people will probably overbid on him because of his potential.) Make sure you get a level 4 trainer and then look for a couple good guards with salaries around 8,000. You probably want them to have at least Proficient-Prominent Outside Defense.
Make sure you are spending money on scouting so you can get good draft picks and then use the extra money you have on expanding your arena.