I have had the googled 135/2.8 for a while. The googles magnify the 90mm so making it usable on a M2 or any 0.58x finder. However it does convert your finder into more or less of a peep hole with minimal peripheral vision, which may be more ideal than a tiny 135mm framline in a 0.72x finder but I know some people will think the contray. It is hefty compared to most M lenses, and with the despised series filter mount it is a real pain to find good filters for it.
I have bought BGN grade lenses from Keh, mostly manual Nikon lenses and they all turned out fine. I have not got any early M lens in BGN grade and refrained from doing so because of the concerns with haze, as BGN is classified as "maybe have scratches and haze that does not affect image quality" but again with Leica glass it could well be permanent. If you are really tempted and resides in the states, fell free to try it because they will offer you a refund with no question asked.
Do stay away from the UG stuff unless you are buying for parts or some trivial accessory, those will be bashed beyond the point of recognition.