What size should I buy?
Until now I ride a Specialized Sirrus Large
I'm 180cm tall with 80cm inside leg (but I'm not sure if I am calculating it correctly as doing it on my own is not obvious)
According to my calculations I should go for the 18'' (which I guess if the equivalent of the Medium) but after visiting 2 bikes stores here, they told me that I should go for a large.
What size should I buy for the Kahuna?
I would advise the 18" frame for you. Having said that bike sizing is very much based on what is comfortable for the rider, so if you have tried a few already, I would suggest going for the size you find the most comfortable.