I think buying him a bush hog is a FANTASTIC idea!!!!!!
Go for the six foot cutter. Not only will he cut more with it in less time, it will cut outside the width of his wheels on both sides, which is really nice if you are doing trails or trying to get closer to a fence, tree or building.
Kubota and LandPride are in a partnership, so it's real nice to be able to get parts for them at the same place. Like everything, it will break or wear out, so support is always your number one concern when buying.
I'm also a big fan on LandPride and highly recomened them. Like mentioned earlier, there are other good brands out there, but none that is better. Just as good, yes.
Buy him the LandPride and you wont go wrong. Have it there at the house for him all wraped up and he'll never forget you for what you've done. Truly a gift to remember!!!!!!
As for medium duty or heavy duty, ask yourself what he will be cuttting and how often? If it's grass, medium duty is plenty. Heavy duty is for sapplings, junk and commercial use. Heavy duty is better, but probably over kill.
Go for the slip clutch if you can afford it. I have a sheer pin in my bush hog and can go months without breaking one, and break four in a single day. They are simple and easy to change, but still a pain to have to deal with it. I have a slip clutch on my finish mower and LOVE IT!!!!!!!! One day, I'll buy a slip clutch for my bush hog.
Congratulations,
Eddie