Being 65 kg as well, I agree with Ian that a Futura 122 would be the best choice for 9-15 knots of wind. However, I'm afraid the North Prisma 7.5 is not gonna make it in true solid 9 knots of wind. You'll need a larger sail, something in the 8.5-9.0 range to be paired with a 44-46 cm fin.
FYI, I can start and sustain planing as of 10 knots with a NP RS2 8.2 racing sail and still be fine in solid 15 knots (Hypersonic 105). With a NP Jet freemove 6.9 sail, a true 12 knots is required and below 10 knots, I'm using a very large sail (11.0).
For starting and sustaining the planing in patchy and gusty 9 knots wind, no wonder, a large sail and a Futura 133 would be more appropriate IMHO.