If you are going to take most of the tractor down to bare metal I would use PPG epoxy primer. I would rather topcoat it with PPG Concept. Both primer and paint are a lot more expensive and complex but I think it's worth it.
When you use a paint with hardener, be sure you use it somewhere where you can get a lot of ventilation even wearing a respirator. The hardener contains isocyanides which is some really nasty stuff. Some of it will go through a chemical respirator. It's really recommended that you use a air supplied respirator with it but their expensive and not very cost effective to paint one tractor. I usually use the stuff outdoors and hold my breath when I'm downwind from the sprayer. Then the problem painting outdoors is wind and bugs. I've been waiting about six weeks on a project I'm working on for the temperature and wind conditions are right. Pretty soon I will have to wait until summer for the bug population to die down.