I assume you are talking about the solid (crystallized?) kind that you mix with water. I prefer to steer away from these and use liquid nutrients that can be used either for hydroponic or soil. These strengths are usually lower, like 4 2 3 or something. I'm pretty sure there is a much wider array of plant manipulating possibilities this way, such as using Big Bud or something, to really get the buds to swell up. Some people use like 12 different kinds of nutrients throughout the grow, depending on what stage the plants are in. Not me.

I highly recommend Fox Farm's Grow Big for veg and Tiger Bloom for bud, and a couple other things sprinkled here and there at your discretion (read up on that). Lot's of other people seem to like those products too. Every other watering or so, you mix these in at 1/4 to 1/2 of the strength recommended for hydroponic applications. That's for if they don't have recommendations for dirt farming.

Fox farm is widely available throughout the internet. Also I would really be surprised if you don't have at least a few of the major brands we have in the states.

Top soil will do, but potting soil is much better. I would mix some perlite, vermiculite, peat, and maybe some quano in it. By the time you do all that, I guess you could just buy potting soil, but then I would still still mix a little vermiculite and perlite in there. Would it be to much trouble to exchange the top soil?