Above advice is very good.
Generally, if you prime the fender and you don't have any weird reactions with the substrate, chances are your topcoat(s) will be fine. The best paint is made at the automotive paint store, is mixed with a reducer (thinner) and has a catalyst added. It is then clearcoated with clear that is also mixed right before painting. This is difficult, expensive, and requires an air compressor and a gun, but also requires that you clean everything up. Drag.
Use a rattle can. Go to an automotive store that carries paint, and ask for help. The auto touchup spraypaint is better than regular spraypaint, has a dedicated topcoat clear, as well as primers specifically made for them. Follow the directions. Watch some how-to videos. This route is cheap, with no cleanup or waste. By the way, you shouldn't have to sand between coats of paint or clearcoat. (Unless you mess up!) Yes, sanding between coats of primer to arrive at the final perfect base is done with "wet" sandpaper, as would be any subsequent "oops" repair of paint and clear. I use 600. 400 is about right for the primer, but too aggressive for the paint/clear.
For the plastic, try Fusion by Krylon. It is for plastic specifically. A supermoto rider I ride with painted his white plastics black with it. He then crashed, repeatedly, on the track and it just scuffed up. It did not peel. Amazing. Do not paint the plastic handguards with regular black spraypaint. I don't think it will work.
What color is the bike/fender?