I'm currently stripping a plastic bumper skirt with Nitromors and soon enough I'll paint it. What's the best thing to prime it with? A simple plastic primer or something else first?

what paint to use really depends on what plastic it is, you would be surprised how many different types of plastic have been used over the years. as well as choosing the right primer and possibly using an adhesion promoter the finish coat may need to have a plasticiser adding if the panel is quite flexible. Best bet would be to take it to a local Auto paint supplier and take their advice.
Or just go for a rattle can of plastic primer and topcoat and see how it works first! :-)