Check out the Camp 9 from Marlin. I have one for the exact reasons you laid out. It uses 20 round S&W 5900 pistol mags and I can shoot with very good accuracy as fast as I can pull the trigger. It doesn't look like the fearsome "black rifle" everyone seems to be so in love with these days, but looks don't count anyway. Looks are for idiots.
With it's carbine length barrel, it's more like a 357 than a 9mm, and being able to hit that fast should be decisive. You can find a nice Camp Carbine for well under $500, mostly around $250-$300. MecGar mags run about $25 each. You do need to change out the factory buffer and recoil spring, but that can be done without tools in under one minute.
The Camp 9 is also more attractive than the Ruger, but that's just my opinion, and as before, looks are for idiots (even me).