I would check out Battlefield Evo. Not the greatest paint jobs in the world, but a good variety of modern soldiers at a reasonable price. I think the scale MAY be a bit above 30mm, though. Aside from that, I am not sure where to find modern minis, especially in plastic.

The official line is that WWG is 1/60.
O scale is not a constant, on continental Europe it is 1/43. This is commonly used for gaming as there is only cover for figures, which cannot duck, behind a tank in the smaller scale, people expect vehicles/buildings to be bigger, and you will not get a 28-32mm figure into a 28-32mm car due to the dodgy anatomy.
Here is the difference:


I now wish I had taken a good img of some 1/43 scale figs next to some game figs, but I used them all in shadow boxes, as I felt like some art. They were normal humans in swimwear from Preiser, and I got a bit suspicious when the prone figures didn't fit into Ainsty/Old Crow medical bay beds, when I compared a standing figure they turned out to be up to a foot taller than a space marine in full armour.
Well 1/60 vehicles fit on the roads, that's all I can say.