I was in a similar situation last season where I didn't think I'd promote, but then after I beat the best team in the league I was undefeated (he was training his rookies, lucky for me). At that point I had clear sailing to the playoffs with an undefeated record so I was going to likely promote through bot cleanup, so you may promote regardless of whether you win the championship or not, so long as you remain undefeated. Promoting via championship is much better, though, as then you get the big attendance boost through the all star break.
Depending on the league you promote into, you will probably need to boost your roster a bit with a couple better players, but it's impossible to know until you get assigned your Div III league. Also, and this was a surprise, the league I promoted into had two teams go bot in my conference, which is unfortunate for the health of the league, but makes it easier to not get demoted. Of course, that isn't an occurrence you can plan for.
Long story short, promote if you can. Don't feel the need to compete with everyone's roster salary from the get go, as there will be teams with giant arena's who can afford the $250-$300K rosters. Best of luck.