If you are sure you will win the league with your current roster, it would be a waste of money to buy two big men. Why spend more on salaries if you are going to promote anyway?
Save that money and use it to stabilize yourself in the higher league next season once you promote. And don't worry about relegation; you only need to find one opponent in your conference and have more wins than him, and then you have to beat the 6th team in other conference in relegation matches. And if you save your money, you will be able to buy those big men when the need arises (so before the relegation matches) and not a week before that. Money is very important in BB, so don't make your team stronger than it needs to be for your goals.