I would recommend a short barrel (18.5 or 20 inch) Rem 870 or Mossberg 500/590. You can always find someplace for a light, ammo carrier like a side saddle and they accept weaver rails for optics if you wanted one. I prefer pump actions over semi auto any day. The recoil of a shotgun makes the follow up shot almost equal between a pump and a semi and a pump usually are easier to use.
I'm an LEO firearms instructor and the Rem 870 and Mossy 500/590 shotguns hold up to the abuse and neglect of police officers. If you don't intend to use it much then a cheaper model such as a maverick or 870 express will do fine. Before you shell out $1,000 or more just go down to Dick's Sporting Good or whatever you have in your area and check out a couple set up for self defense, most are under $500.