Welcome!
As for priming non-body parts, ther are people here who do it both ways. You certainly can prime
everything, but for interiors, engines, suspension, and other detail parts, it's really not necissary. Give everything a good scrub with an old toothbrush and some dishwashing soap, and you'll be fine.
Personally, I only use primer on bodies, or on parts that I'm painting with 'hot' laquers. Acrylics you can paint right on the plastic.