In the meantime, I've gone through Wargames Atlantic's digital design catalogue again and I've identified most of the files I had been eying a couple of months ago. Oddly enough, I can't seem to find the rather essential file of heads wearing Adrian helmets anymore, but I'll keep on searching.
There are several options for the bodies. As I said before; I'm not a button counter, but several items should be clearly represented. One of those are puttees, and those are well represented on these options:
WW1 Canadians
American Doughboys
And this vignette like model has useful headgear; Sgt. Stubby
And for the mounted elements, the German cavalry is a good starting point:
There are quite a few more sets, or parts thereof, that could be useful though, like a baggage train with mules and carts, laden with construction materials, French Foreign Legion, which could donate their kepi's, and quite a few more.
In fact, there are several sets obviously geared towards this very setting, such as pagan conversion sets or priests.
Now for those Adrian helmets