Metal Models Painting

Actually, you can use lacquer on plastic. Just apply in light coats to prevent crazing. My favorite primers are lacquers: Alclad II and Tamiya.

I haven't built any of my metal kits yet, so I have no personal experience. But from what I have read, you can use enamels on metal. For example, automotive primer. Lacquers will work, too. Acrylic will work, but probably won't stick very well and you will have a tough time getting a high gloss (unless you use Future/Pledge floor care).

My plan would be to apply an auto primer to the metal, then finish it off with acrylic.

The important thing is primer.  You do not absolutely need primer on plastic, though many of us do prime it for various reasons.  But metal does need a good primer. Paint does not stick very well to most primers.  The best primers are those intended for painting full size cars.  Ditzler, and Duplicolor are two brands of car paint.  Duplicolor has primer in spray cans that are widely available in auto parts and big box stores.  Also, it is important that there be no oil film on the metal before painting or priming.  Wiping the metal with a good solvent (paint thinner, lacquer thinner, or alcohol are fine for removing oils, like fingerprints, etc).

I disagree with Don's statement of not needing primer on plastic. Primer is a matter of choice but recommended on plastic. However, a good primer on metal is a definite yes. Rustoleum brand primer is best. Find one that's used specifically for metal. As a matter of fact, I use Rustoleum exclusively on all my kits for primer.

I use Tamiya primer, both white and grey, and while not absolutely necessary, I have found that give a nice even surface for the paint to grip and doesn't obscure details. The primer also keeps some of the thinner gloss colors from pulling away from edges or high points on the plastic surface.