Well you've got a great idea, and it's a really good start, just a little advice:
1) By bullet holes, are you meaning large caliber rounds, cause a "bullet" would barely scratch the surface on a tiger. It might do some damage to the zim, but not the armor. Obviosuly the larger the caliber, the more damage it would make. If you are placing holes in the tool box or other thin metal make sure you use a dremel tool on the inside to "thin" the area a little before peircing it, it helps to make the "hole" look more realistic.
2) Grass is fairly easy these days, many companies like Woodland Scenics make products that are very esy to use. Just search you LHS or favorite OLHS for diorama materials. The best sources are often ones that have a lot of model railroad stuff. It is easy to apply, simply dilute white glue (Elmers type) with 50% water, then add a couple drops of liquid dishwashing soap. The either spary it on with a cheap spray bottle, or brush it on using an old brush. Before it dries sprinkle the grass, dirt, what ever around. Wha-la instant grass!
You mentioned you want to do a prairie, sounds like a great idea with a lot of possibilities. Just search for pictures of the plains of Russia, and you'll get a good idea of the colors of grass you'll need.
3) Water is also easy, the same company also makes instant water, that is very esy to use. It can be found in the same section as the grass stuff.
Hope this helps a little...