You'll need wheels, tires, fender, fork brace and I would recommend a better breaking system for the front. You can get Motard kits for a few grand for the popular bikes.
I've had some older Duel sport bikes, 79 Yamaha DT MX 400, 87 Honda MTX 200 R and a 96 Suzuki DR 350 SE. They were all great in their own way.
You can get some pretty good duel purpose tires these days (mostly for street use), add a fork brace and haul butt with out converting them.