RE: A weight problem, which board volume to get? :S
At my weight of about 70kg, the ideal one board quiver would probably be the 100 liter board, especially if one didn't have to concentrate on the absolute minimums. So, at your weight, I wouldn't exceed the 101 range. If I was to suggest a small sail quiver, it would be a 4.2 and 5.0. With your new 6.0 you'll be greatly positioned to handle the 15-30mph range. However, I would suggest thinking about the mean average wind speed, as I believe that it is a better guide in selecting the right sail.
Don't forget to think about some different sized (designed) fins, as they will help you balance a single board quiver in a realistic way, especially if windspeeds pick up to notable levels.