Winter isn't a good time to paint a tractor. I would wait till closer to spring to do it. You need 60-70 degrees to paint. To do the job right it will need to be stripped down to bare metal and be primed soon after being stripped. I prefer automotive paints and would recommend using epoxy primer and topcoat with a 2k urethane. The paint is expensive but it will last a long time.
If you are going to do it now I would start with washing the tractor with a de-greaser and a power washer. If you have access to steam cleaning equipment it would be better. Stubborn grease can be gotten off with oven cleaner.
Then with a sandblaster or abrasives I would take strip as much of the existing paint off as you can. Then I would wipe the metal down with a wax and grease remover and let dry. Then on the cast parts go ahead and prime it with the epoxy primer. The tin parts if any body work needs to be done I would hammer out the dents out as much as possible, wipe it off with the wax and grease remover and prime. Any bondo work can be done over the epoxy. After sanding flat there will likely be bare metal spots again from sanding so I would spray another coat of epoxy primer on the bare metal spots. When that dries a surfacing primer should be used because epoxy primer doesn't sand very well. The surfacing primer sands between coats very well. Once everything is leveled out and primed then it should be ready to topcoat. I prefer a 2k urethane. Automotive paints don't normally have instructions on the can so when you purchase the paint be sure to get a specification sheet on any paints you buy.