Here is another website for heavier ag that may be helpful:
Viewing a forum - Machinery Talk
Your dads current tractor is 120+ hp 2wd, so a newer 90-100hp mfwd, radial tires, etc. with 40% of torque rise may match up well.
An equivalent to a 1466 JD hp wise I think is a 7930 or so and costs in the neighborhood of $125K.
Post back with what you end up getting.