I'd throw in a couple of other components to the equation -- tank size and SPEED.  Are you going to run 5 mph or 8+mph?  Speed eats more Hp.  I like a heavier tractor with 1000+ tank sizes because you have a lot of weight when full to shove the tractor around.  I like the 130 Hp size pretty well for that job.  A JD 4440 would be pretty ideal.  Smaller 100-110 Hp tractors could be very loaded in hills.  I'd rather have a larger tractor run at lower rpms to conserve fuel that something underpowered.