Equipment will depend on what you are painting, the quality you desire of the finished product and what type of paint you are willing to purchase.

Basically the quality of the job will depend on the prep and the primer.  You can put lipstick on a pig but its still a pig.  You can put the best paint over a dent and its still a dent, likewise if you paint over rust, and it will still rust.  (There are a few speciality paints made to go over rust but that's another story.)

For most jobs I prefer to sandblast, prime with an epoxy primer and top coat with urethane paint.  The guns are kinda specific to the type of paint and are best bought from your supplier, at least until you get familiar with what is going on.  To make sheetmetal slick you're gonna need a sander or two and some 'sandable' primer after the epoxy to slick things out.

Email in profile if you want more specifics.