Here's my advice: Don't avoid light winds! If you do, you won't get out much. Instead, get a board that will be fast and fun in both light and strong winds; a longboard with a daggerboard.
If you can't afford a starboard Phantom 320 or 380, I would recommend you get an older longboard (like a mistral equipe) or a new Exocet Kona ONE. A 6.5 - 7.5 sail would be a good starter for a longboard, but longboards aren't so sensitive to sail size as shortboards, so it doesn't matter that much.
I warn you against just getting a shortboard. Even if it's an "early planer" like the GO or Futura and you get the big sails to match it, it still won't work in truly light winds like a longboard does.