How much have you learned on those lessons, are you using harness, planing in footstraps? What volume board have you comfortably used? On a lake, in 5-15knots, at nearly 90kg you'll need a wide shortboard (80-90 cm) with a 10m rig, or a longboard. You'll need some experience to use such big rig, maybe something around 7.5 would be more appropriate in the beginning. Ask what other local sailors are using.
If I were you, I wouldn't rush to buy a new Futura. If you are learning, there's a good chance you'll ruin it (catapults, etc). As a first board, second hand GO or a larger Carve would serve you just fine.