I would look for a used M&P for the same reasons as others have stated. It would need very little prep for long term storage. A major amount of the pistol that's not polymer is stainless steel. Sights, Complete Chassis/slide rails, Sear housing block,Guide rod, Slide, and barrel. There may be other parts, but these I know are. We used M&P's as our duty guns and were subject to the humidity (Florida) and rain exposure for months at a time with almost no maintenance. I never saw one come training time that had any rust on it anywhere.