If you aren't afraid to do some surgery, I like Tamiya kits. Just get out the ole razor saw and start cutting. Actually, if you haven't had any experience/practice doing this, FIRST I would recommend reading Shep Paine's How to Build Dioramas (either one of both editions) which has a really great chapter about modifying/posing figures in realistic ways.
I like Tamiya stuff primarily because of cost (anywhere from $6 to 18 for a set of figures--depending on where you buy 'em) and they are easy to mix-n-match. Also, you get a variety of poses, many of which can--with a little to a lot of creativity--can be modified to what you want. Some people don't like the quality of the older Tamiya figure sets, but with a little work, you can make very realistic looking figures out of the Tamiya offerings. Not to mention, the newer releases of the older sets have a lot more "crisp" molding than the sets produced years ago--which tended to be human-shaped blobs of plastic.
Ebay is a good place to buy a bunch of spare figures and they will provide you with many, many options for practice or parts and pieces for figure modification. You can usually get a bunch of figures for a fraction of the cost of buying new in the store.