It'll interfere with cooling and longevity and the paint only lasts for a few months.
? No it won't. The old cases didn't have mesh anywhere, paint over big ventilation gaps only reduces intake area but a few cubic mm.
No idea what you are doing wrong with paint, but if you can paint a car and have it last a decade, you can paint a case and have it last longer than that unless you are doing strange things to it like using it as a dart board.
The plastics (depending on type, not so much acrylic or polycarb) can be very durably painted with automotive vinyl dye. The catch is that comes in limited colors unless you stray from what is available at the local auto parts store and special order it.
The metal, any stanard enamel paint will be long lasting, but if you want it super long lasting, you can use appliance paint. The good stuff is quite toxic and needs to be sprayed outdoors in a well ventilated area, standing upwind of it.
The rest is just prep work. A glossy original metal paint finish ought to be lightly sanded and then cleaned. Plastics you intend to paint with vinyl dye only need cleaned, should not be sanded because the dye is unlike other types of paints and has poor surface blemish filling because it soaks into the surface of the plastic but the upside to this is it won't flake or chip at all unless you would have taken a big gouge out of the plastic anyway.
If the stock plastics had a paint color over them, that does need sanded of for best vinyl dye application, but in that case I'd prefer multiple very light coats of traditional enamel paint but if talking about the old beige, white, or even most black case plastic, it usually does not have any color coat on top while silver usually does.