Hi Kiteordie,
At 75 kilos and at a beginner/intermediate level, I would recommend nothing bigger than a 12 and nothing smaller than a 9m. This will take care of you in 85% of days.
Of course it also depends upon if you like riding really overpowered on small boards, if thats the case go the 12 or 13, but otherwise you'll struggle to hold onto it.
Another great tip is to sell the 18m on the east coast, it is worth substantially more over there as selling big kites here is like trying to sell icecubes to eskimos, we just use em so rarely that it will gather mould and cobwebs at your home. Airborne was trying to sell brand new superceded 19.5m Lift Pros last summer for $400 (kite only) and couldn't move em. Sell it and use the money for a 14 if you want a big kite for Perth.
Cheers and we'll see ya on the water,

Best winds,
Darren Marshall
Australian kiteboarding School