Gun Test Magazine gave the 870 Express ($345 in Blued Finish) a BUY IT recommendation a couple of years ago.
They liked the trigger and the stock configuration. They didn't like the control placement because the safety and slide release weren't as easy to operate as other shotguns.
If you're going keep it in a corrosive atmosphere the stainless steel would be worth the extra cost.
I have owned one since 1997 or so. Great weapon, it is built to the same specs as the 870 Police. Great weapon for a marine enviroment. I bought it when I used to work a beach assigment on weekends.
You pretty much can;t go wrong with a 870.
If you don't need the special coating, a good 870 Police can eb picked up used for around 325.00.
I don't know what people don't like about the controls on 870, I think they're really good, I much prefer the safety on these than on other guns, its easy to get at and you can always tell when its on because it gets in the way of your trigger finger.
I've had mine for about to years now and love it.
The electroless nickel plating is interesting to me. I bet it would hold up quite well. I think it would make a great all around shotgun. I have an 870 like that but without the nickel plating, and I love it. I think you would be very happy with the Marine 870. And yeah, it should last a looong time.