All of the above. Steering column will need some mods, but it will work. 54" deck will fit the best if you also use the front axle from the 1864.
Not sure how well the D600 in the 782D will pull the 54" deck though. That deck, and all of the other similar newer ones, pull pretty hard. I have a 46GT deck on my 782D and it works the tractor pretty good.
You will need to eliminate the tilt feature. I would make a solid coupling to connect the stub shaft coming out of the box to the rest of the shaft from the donor machine at the right length, and make a new tube of the correct length. I think there is a size of muffler pipe that is the right size. Can't remember for sure.
The Haban is a low-vacuum deck and pulls pretty easily. The 54" deck is a higher-vacuum deck and pulls quite a bit harder. The D600 is not a particularly powerful engine, and Cub Cadet dropped it and kept only the D640 when they went to the next series of tractors.
Anecdote: I mow with a Haban with my 1872 with an 18 hp Kohler Command...it handles that deck pretty well but will get lugged down a bit in tall grass. I can engage the deck at 1/3 throttle or so no problem. Also have mowed with a 2523, which has a 23 hp Kohler Command and the 2000 series version of the 54" deck (same deck and blades, different drive and lift) and it struggles mightily to engage that deck, and it is working the tractor pretty much all the time.
Before you say "But the D600 is a diesel and it has tons of torque!" go find a torque curve for it and compare to a K-series. Really kind of a dog compared to the gas engines used during the same time period. But, it sips fuel though.
Thanks guys for your input, having power steering and a 3 point would be great additions to my 782. This tractor is no longer my primary mower and the 50c deck is in good shape. Always wanted to try a plow on this machine....!