If you mess it up, it can easily be redone. I cannot remember whther the surface is smooth in the corsa b. If not a simple sanding would do or you could use filler and then sand it to make it smooth.
But for painting you'll need 1 tin of primer, 1 tin of paint and 1 tin of clear laquer.
Prime it with a few coats, a few light coats of paint (light coats give a better finish than one big coat) and then a coat of clear laquer. Its not hard to do. Using a heat gun or hairdryer between ach coat speeds everything up. I sprayed the one in my C and was suprised just how easy it was to do!