I can tell You....
You almost have to do research before You know what to buy
you will basically need 0 or S gauge, which is 32 and 22 mm respectively (the gauge of the track)
On30 is HO gauge (16,5 mm) in 0 scale (1/43-48)
try childrens' toys, not railway modelling and don't forget to ask for measurements before You buy.
waggons should be 5-6 cm wide to accomodate artillery with sandbags or vehicle loading. Also keep in mind when judging doors that train doors are usually a bit narrower because they had to accomodate for space restrictions. The main outlines for rolling stock were basically set when they started building tunnels on a large scale in the late 19C. this means that very old locos will have a small boiler and a high funnel. As boilers grew in size, the design had to be altered for length. Efficiency came later with double expansion, oil firing and steam turbines (but this is something you will notice on the models only if You know). Even with pivoting traction trucks, Locos can't exceed a certain length, and that'd be the end of the story (Big Boy and the like).
As a general guide, passenger locos will have very big traction wheels for speed but weaker pull, while goods locos will have small traction wheels for strong pull. There are also medium sized multi purpose ones for shorter distances.
and so on...