You aren’t going to be kiting in 8 knots, at least not for a while. That is hydrofoil territory. On a twin tip, at about your weight, I’d guess my low end with a big kite is about 11-12 knots, and that isn’t any fun. You gain a little bit with a directional, and a lot with a hydrofoil, but those will come later. If your area really tops out around 20 knots, then you do not need a 9m. Ignore that wind chart a previous poster had of “10 to 20 knots” for a 9m. That is absolute garbage. 9m kites are just beginning to get started at 20 knots.
From what you describe, 12m or larger sounds reasonable to me, but I don’t trust you to know what typical conditions are (no offense) so look at what other people are using with twin tips. I don’t know where you got the idea that your beach blows what you say it does, but generally speaking, neither wind meters nor other kiters have any clue what it is actually blowing. Wind meters are often far away, or in wind shadows, and other kiters are just repeating what the wind meter said. You are a little lighter than average, so expect to need 1-2 m less kite than your average rider at your beach. If you see a lot of 10’s, then a 9 sounds reasonable. If you see a lot of 14’s, then no, a 9m will not cut it, and you should not buy anything from your instructor.
Finally, get any brand of kite, just get a beginner kite. Someone is going to tell you to get brand X, and another will passionately argue in favor of brand Y, but they are all pretty much the same, especially with beginner kites. Avoid well used kites, they are never the bargain they seem to be.