I may be able to respond to a couple of the questions.
The 870 Marine Magnum is as good as either its Mossberg or Winchester competitors. I personally find it better than the Mossberg in fit and finish. (No flames from Mossberg fans, please--I like Mossies, too!
I don't believe that Remington offers a choice of either barrel length or choke in the Marine Magnum. So an 18-inch barrel with cylinder bore is what there is. For a defensive weapon, this type of barrel would probably be my first choice anyway.
My 870 Marine Magnum cost me $419, which is over $150 more than an 870 Express HD. I don't know if this is a good or a bad price--my local dealer has had exactly one Marine Magnum in his shop so I went for it while I could. I guess the premium finish and magazine extension add to the price.
Hopefully another poster can respond to the other questions, or improve upon my responses.
[This message has been edited by jimmy (edited September 17, 1999).]