iS111 will suit you fine, you won't fight it in 20+ gusts. As Remi says it's ideal sail is around 7.8, I would say sweet spot from 7.2 to 8.2, but you can use it from around 6.7 to 8.5 no problem.
It is a near sinker with 85kg rider and 8m rig (85+20kg gear = 105), but unless you plan on schloging for miles in 2 knots, don't worry. Have you used ~105l boards (or smaller) before? As long as you are moving (however slow) and you have 5/6 knots of wind you'll get home.