I forgot to ask, are you doing your own powder coat? If so, you might try just one, to see if it comes out to your liking.
If you are having a shop do it, then they should know what would work, and give a recommendation.
The only other thing that I can think of, you need to be aware of coating thickness. This probably wouldn't be a problem on your springs, but can be a real drama when parts need to fit with tight tolerances. The actual thickness of the powder can end up being .030-.050 thick. The coatings also tend to build up more around the edges.
I should have stated more clearly in my previous post, I know that they don't strip every part they coat. They regularly coat black over the factory white powder coat. It isn't usually a problem.