While I don't think that buying a Ltd. Ed. Witch Hunter Inquisitor just for his plasma pistol is quite worth the money, he does have the sweetest gun-slinger-style plasma pistol that
GW has produced. What about using the Cadian Kasrkin Sgt - giving him a plasma pistol and a greenstuff forage cap like the Col.? Or if you wanted to go even easier, you could do the new Catachan Sgt w/ Power Sword and Plasma Pistol and just do a headswap with the Col. And it'd take a little work, but I think the Necromunda Scum w/ Bolt Pistols shown below could be converted to make a really striking Col. Schaefer.