Thank you for the comments folks.
Radu Lykan, your suggestion was an interesting idea and is worth thinking about.
MarcoSkoll, I like your suggestion about making generic NPCs out of the men.
For now, I think I'm going to try to work with the bits I have, instead of buying new heads etc.
The main things that will need to go are the hats and the old school rifles.
Something I want to try is saw off the hats and replace them with scratch built kettle helms. I think (hope) this should be within my scratch building abilities and I think they will have a number of advantages. For one, they'll distance the sillouette from the original model, while retaining the chin strap, so I don't need to worry about removing them and butchering the faces. Secondly, it adds a bit of grimdark to the pieces, in my opinion, while tying the models together to show they're from the same organisation. Thirdly, it hides their foreheads, so I could conceivably use them as fourth generation Genestealer cultists if I feel like it.
As mentioned before, I'm unlikely to game with them, but there's no harm in being prepared, just in case.
With regards to load outs, I'm currently thinking one shotgun, one drawing a combat blade, one las/auto gun and the last guy will get a pole item of some description - perhaps an axe, icon or just a lantern on a stick.
Obviously it depends on the story and power level, but how OP is adding pistols and grenades to everyone? They're great for 40k-ing a model.
Thanks again for the comments folks.
Dallo