I use a 40" Gearmore heavy duty rotary cutter on my
BX2200. It cuts almost anything, including 2" sapplings. A BX will lift about 1200 lb. at the 3ph, 600 lb if the center of gravity is 2' behind the 3ph. A rough cutter is long and thus has a cg that is farther than 2' behind the 3ph.
The manual doesn't give max. weights for implements used while the tractor is stationary: mixer, phd, generator,
chipper, etc. For implements that are used while the tractor is in motion, the max weights listed are about half of what it will actually pick up. This is in case someone abuses the tractor (your nephew from the city visits and decides to take off racing the tractor across a furrowed field perpendicular to the furrows in order to make it bounce as much as possible). With a brush hog attached, the oscillation and G-forces from this abuse could break the 3ph. Listing the max. wt. so low allows Kubota to void warranty claims if overloading AND abuse both occur. You can easily lift more than the max. listed; just don't abuse the tractor.
A 5' light duty brush hog will do if all you're cutting is hay in a pasture. If you're cutting heavy, go with a 36" medium duty or the Gearmore 40" heavy duty. For medium usage, a 48" medium duty is a good choice.