100 grains should be fine for a broadhead.

You'll probably want to look for an arrow in the .340 spine range with a weight of at least 8.0gpi. That will be nearly all of the carbon arrows in the .340 spine range. You are sitting right about on the line between .340 and .400 and whenever that is the case, I always suggest going stiffer with your arrows.

You don't necessarily need the most expensive arrows made, but if you shoot arrows with poor tolerences, you won't notice as good a result as you will with more matched arrows. Using price as an indicator of quailty just to spare you from information overload here.... look at arrows in the $80-$120 per dozen price range. You will find well matched arrows, usually .003"+/- straightness that will perform well on recreational targets and on game.