PM = Private Message. You'll see an icon under people's posts (in the same area where it says Search, Buddy, Profile, etc,. ) that says PM, click on it and you'll be able to send them a private message. You can check your PM's by going into your user settings (click the User Settings icon up where it says P8 Talk, Calender, Members, etc,. )
I'd say Impulse over A5 any day of the week. Fast, cheap, upgradeable, something you won't look like an idiot playing with if you go to a tournament sometime in your future...