hey Duff, I kite at St Kilda too.
You are starting as Autum approaches and the winds peter out (Which is a bit of a joke because there hasn't been much wind to speak of anyway). This is when I started almost exactly one year ago and it's been a frustrating learning process ever since due to the fecal season.
Wouldn't have been as bad had I bought a larger kite. I have an 11m Argo (92kg and 140x42 board) which is supposed to be a bit gruntier at lower wind speed but the point is that you have to be reasonably competent to keep things going with a marginal wind/kite combo. When the time comes to making your first runs, its easier when you have reasonable wind up your kilt and good but manageable power from the kite and most of the time the 11m was shy of this.
In the current conditions and your 98kg's I would think that you'll be needing every bit of that 13m BUT, as with the stock market, the question is: will the trend continue?! I would have been fine with the 11m had Melbourne graced us with her normal summer sea breezes but la Nina fixed that with a wet and cool interior. How about ringing the BOM and finding out if la Nina will still be around next season before you buy