I have scratchbuilt some in 1/700, using styrene strip and stock shapes, but only one-offs, as per the pics below. If you are looking at making huge or even moderate-sized "battle" dioramas, and don't want to spend the rest of your natural life making them, I think that resin casting is going to be the way to go. Make a good master and then pop out as many as you need.
Of course, there are ample vehicles available, for some of the countries, in the desired scale, but that gets real expensive, real quick. Below are two of my efforts, a daihatsu landing barge, and some railroad rolling stock:
