I believe Remington 870's and Mossberg 500's are the workhorse shotguns of American law enforcement. These two shotguns can be used in many applications and have many aftermarket accessories and options available. One reason that these shotguns are so versatile is that by opening the action halfway and unscrewing the tubular magazine catch, the barrels can be switched out, often with just a simple tug. This allows you to use the same gun to shoot sabot slug, buckshot, target load, and breaching rounds. I believe both of these guns run in the $350 to $500 range brand new and may include a second barrell.
I'm partial to the Marine Corps' Benelli M-4 but the civilian version carries a price tag of around $1400. My brother-in-law recently bought a Mossberg Maverick for around $120(?). It felt like a much cheaper quality firearm and only held two rounds in the magazine, but some of the same aftermarket parts for the Mossberg 500 fit the Maverick. It may be an excellent starter shotgun if the barrels are also interchangeable, I would definitely do my research before buying one though. I doubt that they hold much resale value.
Just remember not to shoot a rifled slug through your mod. choke barrel, it severely degrades the performance of your choke. I've run into several military guys and average joes who didn't know what a choke was or how it worked. At one time, I was that guy. hence the word of caution.