Im not convinced that the power tools are the way forward. What if your oppenant unplugged you!! maybe battery powered...

a 5" would have to be the best choice though. not too big (9") not too small (4"). i would use a flap disk on mine. Not the instant lacerations, but it would rip off bits of arm and leg. Cutting disk is a clean cut. A flappa would dislodge chunks.

Power tool of choice, would have to be a die grinder with tapered steel burr for good ole stabbing action!!

Oh and i dispute that car parts woundt be enough. I reckon a bonnet to the chops would cause some pain, maybe remove teeth. Difficult to wield mind you, but an effective bonker to the head.

I reckon a steel fan blade setup from a thou, used ninja star style might be fun.

Perhaps setup mad max 3 cage style