I'm no expert, but if you go with the semi-auto, here's a suggestion:
Get a couple extra magazines, and just swap them out from time to time. I usually shoot the ammo I keep in my self-defense weapons once a month, but that may not be necessary. I have shot pistol ammo at the range that was many years old, and it caused no problems. It had been kept dry, and had no apparent corrosion, and the gun functioned just fine. I know it's not what you meant, but owning a gun is not a casual thing. Be safe, get as much practice as you can, and good luck!