There are only two things I would be careful about using those sanding drums in a drill. The first is that it is purely a finishing system since you can't really push the work onto the drum without worrying what you are doing to the drill bearings, which probably aren't built to take side loading like that.
Second, the drums that I have see were slighly soft, foam like, if you are trying to make the slabs flush with the tang, and you are sanding in line with the tang, as a drum does, there is a slight danger of undercutting the softer handle material either side of the steel. The wood cuts faster than the steel and a soft backing to the sand paper doesn't keep if all dead flat.
I don't know how those would work, I am only going on the drums that I have seen and they might have been different.
I really wouldn't spend the money at this stage on those power tools. A 1/2 - 1" wide x 12" long flat bit of wood or metal as backing, with either wet and dry, or premium AL oxide paper will clean things up really well. Shop roll abrasive, in 120 grit or coa rser will round the handle nicely and you can do an amazing amount of shaping with a long 1/2" square b'stard file. Being only 1/2 wide you can use it to cut concave curved areas, like the waist on the back of a Woodlore type handle. All the above can be worked at 90degrees to the spine and cut wood and metal at the same rate, so keeping it flush and flat.