How shiny do you want it? :)
Seriously, almost anyone can paint over perfect metal and get shiny.
No one can paint over ripples and get shiny.

With paint work, it's all in the prep work.

If there's no rust, no need to go to the metal.  At the quickest and
dirtiest level, scuff the paint with 300 grit or so, and prime and paint
over that.

When you're sanding, if you don't think it's smooth enough, you're
right.  If you think it's smooth enough, you're probably wrong. ;)

It depends on how many hours you want to invest in it.  I spend about
two hours sanding my race car before paint.  I'd probably spend 100
hours on a show car.  A pro would probably spend even more....

Ray