Here's my thinking on this:
single-action, external mix - clear coats, where you want even coverage but fine atomization isn't a major concern
single-action, internal mix - primers, metallics, and color coats, where you want even coverage and good atomization...note: a separate brush for metallics only, or metallics/primers, wouldn't be a bad idea
double-action, internal mix - detail work (mottling, weathering, shading, etc)
I think you could support this array without breaking the bank it you shop carefully. Paul Budzik has a video on choosing the right airbrush that is highly informative and fascinating