Old gravely's had reverse on them if you can find a sickle mower attachment. But the gravelys are so heavy that they can get away from you on a steep bank if you're not really careful.
People have gotten away from sickle units because of having to maintain the cutters and because of the vibration. Plus they lay the material down instead of chopping it up like a brush mower.
One thing you should add to your list is how much traction the unit will have. I think bigger tires and more traction is good to have.
If you don't have a lot of material to cut, a weed trimmer equipped with a metal brush blade works well on creek banks. Very light, cheap, manuverable.