The point of primers is to "grab" the plastic giving you a good surface to adhere the paint to. Which is why you don't find acrylic primers, they aren't aggressive enough. However, if you are patient, and willing to give the paint time to cure (2-4 days depended on temperature and humidity) just use one of the flat light grays. Once acrylic paints are fully cured, they are a very tough surface. Make sure that you have washed off any remaining mold release agents.
A properly cleaned surface doesn't even need a primer coat, though often they show minor imperfections that can be cleaned up before investing time and effort into a complex paint job.
And yes, you can paint acrylics onto a fully cured enamel base.