Here are some of my thoughts in no particular order.

The shotgun market where I am in Ohio is terrible right now. Everyone bought up everything and I feel like manufacturers are rushing to get product out. I could not find a new Mossberg 500 12 ga. field gun for a coworkers son for 3 months. I also believe that you should try and find a Rem 870 used police gun. I got a Highway Patrol model with extended magazine adn side saddle last year for $299. It is marked up on the outside but like new on the inside.

Differences between Police and Express off my head from what I remember from armorers school. Express made on assembly line, police is made start to finish by 1 guy per gun. Each police is patterned and tested, they test and pattern 1 from the batch of express. Express is baked enamel like something or other and the police is parkerized. Express has MIM extractor and not designed to be kept loaded magazine spring. Police is machined extractor and extra power mag spring designed to be kept loaded. There was something else in the trigger mechanism too but I would have to look it up. I have told firends to handle an express they like and just replace the extractor and mag spring if they want to do it cheap. I would never buy a new police shotgun myself unless I could get it at dealer price or something.

Early express shotguns did not have plastic trigger guards and the magazine tube was not crimped, I have one in my safe. However they did start crimping them pretty soon but still kept metal trigger guard until a "few" years ago.

Mossberg 590A1 fixes two of the three biggest problems of the 590. They have a metal trigger guard and a metal safety. I have had plastic safety mossbergs engage while shooting buckshot in training. Never seen that happen with the metal safety. Last problem they have is spring not designed to be kept loaded in my opinion. Not sure if A1 is or not because I replaced my with a Wolf that was designed for that right away.

I use a Browning Maxus to hunt ducks and I love it. I don't think I can bring myself to trust a gas operated shotgun for defense though. The horror stories might just be stories but I have seen semi autos fail in the field.

Putting ghost ring sights on a remington 870 is not that hard. Just about any gunsmith or machinist can do it. I myself like the rifle sighted smooth bore slug barrel on my OSP shotgun almost as much as my ghost ring 590A1.

Ohh yeah, you can drill out the crimp on the Express 870 so you can put a mag extension on it, I have done it myself. You just need to sand the inside good to make sure there are no burrs.