Ditto what Darla said.
When I first started doing helmets & such, I actually used Createx t-shirt paint & rattle can urethane clears. Just be sure to layer on the clear in smooth, even coats... Thinner is better.
When you're ready to move up to "real automotive paint", you have a LOT of choices. I would recommend Auto Air colors. I think they're great for someone who is not used to shooting enamels or traditional automotive urethanes. They're water-based & clean up very easily. You can also spray them in your home, because you don't have vapor/fume issues. Much safer. (But still wear a mask, please!)
As for the different needle/tip/head assembly sizes, these are really designed for use with different materials, not to achieve finer or thicker lines. Like Darla, I rarely use anything other than a #3 needle/tip/head assembly combo on my Paasche VL. The exception is a #5 for metallics, so that the larger particles can pass more cleanly through my brush. Never needed #1, which you could use with very thin mediums such as inks.
Hope that helps some!
Have fun & good luck!
Marge