1971 Challenger sanded down to bare plastic. It will be getting blacked out, R/T style. Hopefully using the Summit satin black paint I already have with a little texture coating added to match the tail light surround before the final coat. Yes I know R/Ts were bare plastic, but I've seen some where mold lines were very noticeable and don't want that.

Is any kind of special primer or paint needed?
Adhesion promoter?
Will regular automotive paint work or do these move enough to need flex agent added to paint?
Will very thin body filler in some imperfections okay?

if you want it to last, pick a paint supplier and call their area Rep., tell him what ou are doing and follow his instructions to the letter DO NOT take the advice from a paint store counter person DAMHIK this

Bondo is fine on top of plastic welded with heat, I would use a sandable primer over the repaired areas. 30 years ago I used laquar primer/ surfacer and paint with no flex and those grills still look fine, both black and argent. You can get black surfacer in a $5 rustoleum can to put over your bondo. Be careful with the latest whiz bang flex and paints as some of it shoots too thick and looks like sheet when done. I see no reason for adhesion promoter if it is all sanded. Do a test on something other than your grille and see how it "looks", I still use laquar for the "look" but am sure something better is out there. Your summit paint is probably fine but I would shoot some primer on something then the summit paint to be sure they like each other, a repaired grille is the last place you want a paint reaction.

SEM Trim Black 39143 is what many use for the black plastic on Charger grills. I don't know about the texture, but SEM has a textured black coating for plastic, too, SEM 39853 Black Texture Coating

The 71 Challenger RT grill was not satin but more of a low gloss black un painted plastic so to look more OEM like you may want to paint it with a more glossy black, not satin. Im doing the same grill and searching for a more correct look. It was not textured black , just bare kind of low gloss plastic . Maybe a low gloss SEM paint ?