I would say my IV league was a bit above average, we always had a few teams which were ready for III. Here is what I was running with when I won my IV title.
$17K PG balanced, but with 14 OD
$28K SG not very good inside (inside skills were 3-6) but very balanced outside game, outside skills were all 11-14
$12K SF training, nearly perfectly balanced, moved him around a lot during the season for training/match-ups. All but one of his skills were 11-8 at the time.
$22K PF Balanced, had 7 OD
$38K C Again, balanced had 6 OD, I also bought this guys that season to put me over the top, made the finals with this same lineup and a $16K C the season before.
So that is what it too me to move up to III. My roster was all $15K or less (starters were probably between 8K-15K) when I promoted and I did fairly well (I have yet to play in a regulation match), but I also am (obviously) a huge fan of balanced players so my roster always seems to play better than their salary.
Also, make sure you manage game shape, your starting C played 144 minutes last week, that can't happen as promote through the ranks.
Good luck