I was rooting around some old property lists a while back and turned up a sheet that had Win 1200's on it. They were "low bid" and thats why our outfit got them, they were cheaper than 870's at the time. They were also outside of policy by 1990, and not reliable to boot. We were much relieved to get 870's.
A vintage Wingmaster like you describe is a great shotgun. All steel and made prior Rems current up/down cycle of quality. If the finish is that bad, Duracoat or some other refinish may help you live w/ it.
Fixed choke Rem barrles are not that uncommon. About every shop I've ever been in has several gathering dust in the corners. You ought to be able to scare another up priced right.
I would not let the 3"v.2.75" issue spook me. Modern 2.75 mags are serious ammo and do not give up much to the 3" shell.
Get the Remington!