The character power boosts cannot be purchased until you have a character up to 950 base power. So you would need to play the game, and level up a character, then you could purchase the boost for the next character. In all honesty, once you have a character to 950, it is very easy to level up other characters. As the power level of your weapons from the highest character, will affect the drops for the others. It only takes an hour or two to get a second or third character to 950.

You need to have a character with a light level above it. But even then you can probably just stock up on prime engrams and open them with the new character to save the money.