As much as I like my little L175, I think a five foot box blade would be too much fun for it if you try to fill it full. I don't really know, mine has turf tires so it would lose traction and spin before you sould get the box full of dirt.
AS for the rotary cuter -- The L175 will handle a five foot finish mower fine, but I do not think it would work very well when mowing heavy grass/ weeds and brush with a rotary cutter/ brush hog. I use a 5' pull type brush hog behind my Allis Chalmers WD (roughly 35 HP)and it works the tractor pretty hard in heavy grass/ briars/ ragweed in my pasture, so I have never tried to use it behind the Kubota.
Hope this helps,
Robert