Hey Dimitri
Three gorillas is plenty of money to get yourself close to top of the line everything, however this may not be the wisest move. Most people wear their first kites out rather fast as they tend to crash them a lot more than experienced kiters. I certainly did, I lost track of how many patches I had in it.
Instead you might want to consider a kite one or two seasons old, second hand, and save your money for an upgrade once you aren't crashing it so often. Just a suggestion though, feel free to ignore it.
The difference between a 9m and a 13m is.... four metres of sail area. More wind, less sail. More sail for less wind. There are different handling characteristics of different size kites but this is merely a function of size. More significant is the difference between a 13m 'C' shaped kite, a 13m 'bow' kite, a 13m hybrid, a 13m bridled foil or a 13m unbridled foil. Do a search here, there is way more information (and argument over which is better) than I could regurgitate now.
But probably the most important thing, get some lessons. Kiteboarding is dangerous and if you don't know some key safety info then you could get in a lot of trouble, or dead.
Welcome to the asylum.