I have a M645 and the original 45/2.8 (with 77mm filter thread) in addition to the 80/4 macro and 110/2.8. The 45mm was bought used; it can't have seen much use as it looks almost like new. Most likely it has been sitting on a shelf most of its life... However the aperture is somewhat sticky - it is possible the change the aperture, but the blades move slowly. Usable in manual mode, but not in auto stop down mode. Not uncommon for lenses which have seen little or no use, so this should be checked as a cleaning may be required (I assume this is valid for all lenses). Otherwise the 45 is a nice, sharp lens - but it's not exactly small and lightweight
If the condition is good and the price is right, go for it!