There is a 'plasticiser' added to 2k paints to allow it to stay on Polyprop bumpers. lets it flex + temperature movement. I believe the plastic surface may require an acid etch primer then undercoat and if it is 2k the 'plasticiser' will need to be added to it. Personally I'd keep 2k epoxy for the steel work as it is quite thick and you need a thinner flexible coating on a trim or bumper. 2K Polyurethane/acrylics are fine and can be mixed to give nice thin even coats.
A 2k polyester primer filler is also available but again it is too thick and may crack off. Don't forget you can put 1 part over 2k but not the other way round.
Anything about BXs