Swanndri's are excellent and fairly water-proof.
I second the need to stick with "Merino" wool if at all possible, as that type of sheep wool is FAR easier on the skin than any other type of wool. I've found several thin "Merino" shirts on the evil-auction-site for as little as $4.99 brand new!
If you can't find Merino at Salvation Army, look for lamb's wool. It's "almost" as comfortable.
As far as undershirts go, I'll easily wear Merino next to my skin, but I haven't found anything to be better than the polypropylene underwear with silver fiber threaded throughout (i.e. X-Static/X-Scent). It's:
anti-static
anti-bacterial (phenomenal odour reduction!)
thermo-regulating
moisture wicking