I've been to Switzerland (Zurich and some tours around), and I can appreciate how different things are there and Europe in general.
Yeah, for the fear of craft store paint being less durable / having lower quality nozzles, I would buy supplies from automotive supply shops. They should have higher quality sandpaper, good quality rubbing and polishing compounds, and good quality paint. Keep an eye on what solvents the paints use. For example, don't put an acrylic clear coat over oil enamel--it will dissolve the enamel. But you can put an enamel clear coat over acrylic base coat. The general rule is to mix like with like, so acrylic paint + acrylic clear coat, or enamel paint + enamel (oil) clear coat.
As for how long you should polish, there's no right answer. You won't get a perfect finish anyway, because you're limited by spray cans and lots of curves on your keyboard case (plus lack of experience--that even goes for me vs. professional car painters), so just do what you think looks good and don't worry about it too much.
The more clear coat you put on, the thicker the clear layer is, and the more you can sand off with very fine sandpaper (2000-grit) to smooth out the surface to mirror shine. Problem is that too much clear coat can be bad: if you spray too much and too soon, it can turn milky, can become dull, or can even give your paint a yellow shade (depends on clear coat, some are better quality and resist yellowing better, but nonetheless, if you spray too thick, most likely you will see the yellowing).
For computer cases, you can safely do multiple thicker clear coats and sand them later, as surfaces are generally flat and extended, which makes polishing easier. For keyboards, I'd stick with a few thin layers of clear coat. There's a difference between a wet shine look from a thinner clear coat and a wet shine look from a too-thick clear coat where you spray too much at once, and the paint is all runny. As you practice, you will get a feel for "too much" for a single layer. Remember that you don't want too much paint in total, as it will inhibit the curing process.
If there is very little clear coat, I only go very lightly with 2000-grit sandpaper in a circular motion, applying very little pressure and keeping the surface completely wet with water--just enough to get rid of the roughness, and then use polishing compounds. That means that the surface will still have some of that "orange peel" texture to it. Which is not necessarily a bad thing--it can suit a particular look. Sometimes I'd do just a fine couple layers of matte clear coat, in which case I don't really polish much at all. And sometimes I'll use more clear coat and polish longer for a more mirrored look. Just stop often, wipe your surface, and make sure you don't over-sand / over-polish. It's very easy to eat through the clear coat and onto the base paint, which can actually make it much worse. For example, if you use metallic paint, if you sand the paint under the clear coat, you will sand off the color part, leaving a lot of the metal flake, so that spot can look more dull and more silver (or whatever the sparkle component color). That goes for any paint that is not a single uniform color throughout. It's better to leave the surface a bit rough and deal with it than end up with blotchy color variation, because fixing that later will be near-impossible, unless you redo the whole thing.
