Questions on interior trim

Without question it's all in the prep..
Use rubber gloves and keep your oily fingers off of things.. I usually use a tsp or some kind of cleaner that definitely gets every known oil off of it... After that I've used just cheap Bi-Mart vinyl paint...
As far as your plastic pieces I would just put out they wanted ad and get some good ones... After all this is your dream car...
for instance I have a near-perfect dash pad stashed away that I got when the getting was good but I won't put it in until the car is painted... The one that's in there now with some imperfections will do...
maybe like you're doing do what you can with the ones that you have but keep your eyes peeled for good ones.. and pony up the price that they want cuz they're not grown on trees especially if they're really nice...