I'm a little confused by parts of your question, but I'll try to help. Using "flats" for your woodland is only going to work if your woods are viewed from the front or back. If you are generating views from any other angles, it won't work. If you want to make a large terrain for your basic ground plane, use the settings for gigantic or planatary size (it's the little 9 box grid in the terrain editor). My thoughts on doing a big scene with several things happening would be not to try several different renders ... unless you plan to composite it into a panorama or render it as an animation, but to consider a bird's eye view.

Could be worse, could be raining.