it seems that plastic gw minis are the best choice - you will make quite cheap and consistent unit out of 1 plastic box, no matter if you choose flagellants or other models. I'd personally go with Empire or Bretonnia army for this purpose, I can hardly imagine dwarven/elven ghosts, not to mention skaven or chaos warriors

Of course you might use a combination of minis from 2 armies and say that these are ghosts summoned at some old battlefield

I can't remember any company making a wide selection of ghosts, apart from Reaper. But, buying many metal models will make your unit much less affordable

you could perhaps avoid these costs by choosing something like this, as each would make for a complete base

