I recently bought the Viper 80 (which I sold) and now have the Rio Small (the 75cm wide).
I bought both to teach my girlfriend at 65kg, I would say the Rio Small at 180L, 75cm is an excellent choice, especially for fickle winds on a lake. The Viper 80 is unnecessarily big for someone your size.
The viper 70 and 75s would both be good, keep in mind these are quite heavy to carry around, put on the car by yourself etc, the viper 70 might be a challenge at first, especially if you don't have anymore lessons.
I would avoid the big Go if your learning on a lake where the sailing conditions might be predominately less than <6m/s.
For lake sailing a 5m sail is probably a good compromise, if you have to choose just one sail. I taught my girlfriend on a 4.5, in the UK.
What about joining a club, or renting for little longer until you have more of an idea of the type of sailing you want to do ?, is the Kona class popular in Norway, I understand it is in Sweden, the Bic tecno might also be a good choice, depending on what you want to do.
This might be some help
www.2windy.com/index.php?article=windsurfing_norway