PTO horsepower with brush hogging only really matters on the material you're mowing. If its thin dry weeds that wont lug the tractor down, then a lower PTO HP rating will work fine. If you're mowing tall green grass, then you're going to want something with a bit more power to get through it.
Having mowed both with my 45hp tractor with a 6ft rotary mower, that tall green grass gives it a workout! Usually moving slow, range 2, gear 3 or 4, I have to slow down in the tall stuff.