Personally I was looking at the Precursor Knights. You'd have to replace the mace with a sword (or maybe not because the mace is very kewl) and their heads seem in keeping with what you want and also can be easily replaced. The shield hand would be where I put the pistol.
But as you say, most likely the major obstacle with Warmachine is that the figures are almost certainly very large, though I don't know that for sure. I've always liked the Warmachine minis but could never figure out what to do with them...
Wish you good luck on the project (it's a great idea) and am looking forward to what you come up with.