Used Oil Based Primer on MDF - What kind of paint should I use?

I made a component rack out of MDF and I put 3 layers of primer on it (Kilz Oil Based) and I now want to paint it black.

What level of sand paper should I run the Orbital Sander on the last layer of Oil Based primer that I used?

Also, what kind of paint should I use as far as oil based or "primer and paint all in one" or "water based" (I want to paint it black and I am going to use Cigar Foam paint rollers to paint it black)??

Thanks!