I'm not very good in dutch, so I'm gonna write this post in english. Sorry for that
But I can read in dutch (with some help of google translater ).

I'm moving to Amsterdam soon, and I wanna start kitesurfing here (IJmuiden, Zandvoort).
I have experience in snowkiting (I mastered some basic backrolls and grabs). And I have very little experience in kitesurfing.
I have 2 bow kites right now.
Cabrinha Omega 15m - I use in from 8 knots to ~20 knots
Liquid Force Assault 12m - I use it from 20 to ~28 knots
We don't have winds stronger than that here :)
I'm 80 kg.

So, could anyone please advice the perfect kite sizes for me.
Oh yes, I'm intrested in old school jumps, with great hangtime, etc
Should I bring my bow kites with me? Or should I sell them, and buy something smaller in NL, maybe 8 12?
Thanks for advices.

In NL we have all sorts of wind!
In summer you can use your 15m/12 a lot but when fall kicks in you should have a smaller kite 8 or even smaller then that.
Right now the wind is either off or on (6,7,8m (30 kts))

i would only buy a new small kite the rest is fine.

What I heard some time ago, is that the winds in the mountains are pretty "thin" which means that you can hold your kite longer in stronger winds, but the wind speeds you name in the beginning of this topic, are way to strong for such kites here in Holland.
If I were you i would go for a 9 12 set, and it depends on you whether you would like to buy fresh kites or not.