I would do whatever it took to get the rest of the paint off. Sandblasting works the best but if your not equipped you might use a silicon carbide paint stripping wheel on a angle grinder. Depending on which brand paint stripper you used a lot of them contain wax to prevent it from evaporating so it would be a good idea to go over it with a wax and grease remover to be sure.
You don't normally have to sand the metal. Only if there is places to be repaired or rust to remove.
There is really no reason to attempt to do a good paint job and use a cheap primer. I'm a firm believer in using an epoxy primer. Etching primers work well on a car but the paint on a tractor is subjected to oils and chemicals which the epoxy is better able to handle. Also if you would topcoat with a 2k urethane it would also withstand chemicals as well. Yes, these finishing products are expensive however it's a lot more labor intensive to paint a tractor than a car and I would want the finish to last as long as possible.