I got to thinking about what models from the wargame would be the most useful for RPG players. Many typical fantasy models would work fine in the IK, but not too many fantasy models have the kind of firearms or mechanika type equipment that is iconic in the IK. So here's my list of helpful Warmachine models for use in the IKRPG:
Sea Dog Crew
Short list, I know. Obviously not a comprehensive list meant to cover any possible encounter. BUT!
Winterguard are a good proxy for any generic "military unit with a gun." Obviously they're Khador themed in appearance, but who doesn't use "stands for" models in their RPG games, right?
Sea Dogs are great models to represent any non-military group. They're variously armed, so you get the whole rag-tag feel, they don't have uniforms, etc.
Mechanithralls are great IK-themed zombies. Obviously there's more to the Undead in the Iron Kingdoms than Cryx, and your 2 bit Necromancer operating independantly out of some abandoned manor house will probably be simply raising skeletons and zombies and leaving things at that as opposed to running a full-fledged chop shop/meat house complete with steam augmentations, but if you DM much I'm sure you have a pretty big handful of skeletons and zombies already at your disposal.
I tried to keep to units that have obvious Iron Kingdoms ties - what with guns, steam-stuff... Feel free to critique and add your own to the list.