If you're like me, say, paint several pieces at a time,
with each piece painted individually including mixing paint in small batches, In other words, everything painted but done piece meal,
You'd likely require 1.5 gallons of paint to cover your tractor in one color. You'd require about the same amount, for the primer as well. Once you decide on a primer color, stick with it,, don't change that base color.
I'd posted this info in one of my builds, paint amount, mixing parts of hardener to parts of paint (Required in my mind) and thinner (although I don't recall the amount if any) Measuring cups,, absolutely mandatory.
You don't require a very expensive paint gun,, (all that means is, you'll spend a huge amount of time cleaning it) Get a cheaper one < $50 and chuck it after a whiles. It's not the gun that gives you a nice paint job,, it's the prep, the paint mix parts, and your learning ability. (It's not hard, but it does come with a bit of hard learnings, no big deal, let it dry, clean it up, and shoot again)
Case IH paints is a tough paint, and fairly decent to work with. (difficult to beat it)