To be legal, I believe the barrel length has to be at least 18 inches. I suggest looking at either a Remington 870 police model or Marine Magnum or a Mossberg 590. These models are reliable and can usually be found for a good price. In CA, easily under $350. You can add goodies later as you please. e.g. Side saddle carrier for extra rounds, extended mag tube.
Hope this helps.