Try and get one with little machanical damage and no optical. It does not have to be pristine mechanically, just not badly bent, and able to take filters.
Check the shutter is ok slow and fast speeds.
P.S. Use a hood.
I can't say I agree. A lens with a little WEAR on the paint finish of the barrel may be OK, but any signs of damage indicate a probability that the lens in question has been dropped, in which case all bets are off regarding optical performance!
When I recently bought a lens from ffordes photography (reputable dealers in the UK) and asked this very question they explained they and other dealers had tried to clarify exactly what the letters meant and found Mamiya unforthcoming - except to say it referred to the period in which the lenses were coated.
A while back I was reading someone's ebay listing, and it said that the W indicates that there is a wider aperture range between the click stops. I believe this might be true because I have a 110mm non-W and a 180mm W-N and the 180 definitely has a wider range between stops (just like the K/L L and K/L L-A lenses for the RB). I don't know what the difference is between the W and the W-N though.
Anybody got any experience with the Sekor Z 150mm f3.5? I'm thinking about getting one. Also, I can't decide between the 65mm Z lens or the 75mm. Any recommendations on that?