"Try before you buy" is the motto of the powerhouse, and you best make use of it.  You can do two wonderful things here:

1 - Pick a power and then test it out in a training room to make sure you like it - if not, you can replace it with another power as long as you're still in the powerhouse.
2 - Respec your character - respecs in Champions work sort of in reverse of how you've been building your character.  You pay some resources (money) to undo the last thing you added to your character (power, stat, advantages, whatever), and keep undoing the ones previous to that until you get to the point where you've undone whatever you don't like.  You can only respec up to the past 10 choices you've picked. (Monk's note: and it gets more and more costly the further back you undo)