What type rust converter should I buy?

1.)My black honda civic has a small rust spot on it, what type of rust converter should I buy to get rid of it? And
2.) there are a bunch of different brands form $20-$100 are the cheap ones useful and
3.) would I be able to just leave the rust converter on the car without painting over it since it is black anyways.

You don't need to spend more. They're all pretty much the same. The black is actually a primer. You should paint it.

POR 15 is good enough.