There is no point in sanding that fine. 320 is more than fine enough, 220 would be ok for pre-prime sanding. Hi building primer would fill that just fine, plus it gets a better grip on the substrate.As for finish, if you use "bondo" on the whole thing and sand with 400-500 grit then you can take the whole thing to a body shop and have them prime it and paint it with automotive paint. Use a base coat and clear coat system, that way you can water sand with 2000 grit and polish it.
If you have the capability to spray, there is no reason to pay a bodyshop....even if you could afford it, or find one that would do it. This is really all about the hand work, not the spray anyway. There is no reason not to use Lacquer, it will look fine. It was done that way long before modern 2-part automotive paint. You can still "sand & polish" the Lacquer clear, though 2000 grit is too fine for that too. Paper that fine just "follows the bumps", you have to "cut the top off" of the final coat with something more like 1000-1200 grit. You can then step-up to finer paper if you want to. It will shorten the polishing time.