I just buy wool shirts out of LL Beam, or woolrich, or somebody. I picked up a light weight set of wool camo about 10 years ago from woolrich that's great. I don't see it anymore. The pants are 20 oz and the shirt 15. Here are some that would work under your camo. Anything from 9 or 10 oz to 15 won't bulk you up too much. Buy it big enough to layer. You can also get light weight wools from Pendelton. These in the link are washable.
http://cabelas.com/cabelas/en/templates/product/standard-item.jsp?id=0022572942019a&navCount=1&podI d=0022572942019&parentId=cat601740&masterp athid=&navAction=jump&cmCat=MainCatcat4700 76-cat601740&catalogCode=UG&rid=&parentTy pe=index&indexId=cat601740&hasJS=true

I have a heavier weight one in Forest green that's great too. I picked up a midweight Jack shirt from woolrich too that's good for moderate cold with my undies layered and my advantage camo wool outside. The only thing I should add is some type of windstopper shirt layer.

More I know nothing about
http://www.bizrate.com/menspants/material--wool/products__att259688--267483-__keyword--camo%20pants.html