What I did to eventually win D3 was to build a team of 10 players with salaries in the 15-30K range.
*cough*and then you brought in a superstar guard that's running roughshod over the league*cough* ;)
But yes, a team with quality and depth is a good way to get through IV and to III. A team built around a single player or two can be crippled with an untimely injury or two, and will have more work managing GS, plus is susceptible to bleeding points when backups come into the game.
The way I've looked at building the team when I was in the lower divisions was that I'd bring guys in as starters, and then as I moved up I'd bring in newer guys and bump the starters to backups, and then eventually to deep bench roles and finally sell them. So for you, tough, when you pick up guys for IV, try to get guys who can help you out in IV and that you can trust to play some minutes in III if you get there, which means as few holes as possible in their game, reasonable salaries and hopefully not too old. A lot of people sell and replace their guys more frequently than I do and there's nothing wrong with that, so you can obviously do that as well.