I like bigmoe's advice. Just buy the tractor you've seen pull & save the experimentation time. You should be able to buy most tractors based on 50 cents on the parts installed. Then, change little things as you go along.

I'm not really sure you can "compete" with a pulling tractor that's also a farm tractor in either of the organizations you're looking at, unless you're talking going into Barnyard class pulling. But, none of the organizations support that class. Slightly different rule changes per event. We pull in both organizations, though not in your intended class.