I've spent a long time listening to folks yelling about "plastic this" and "MIM that" in 870 Express shotguns. Had a customer in one of the shops I used to do some work for who just HAD to have a Wilson 870, nothin' else would do. They ordered him one. It came in.
He was dismayed to find an Express gun with a plastic trigger plate, and to find the big- head safety he had ordered in the carton with a note that said due to liability issues (this was J-hook safety days, remember) he'd have to install it himself if he wanted it in there.
Dude was almost apoplectic. Manager ordered the old style safety detent and spring from Brownells for him and got me to install them along with the big head safety for the customer while he watched, right there in the store. It satisfied him and he took the gun home.
My advice FWIW, buy a good used agency trade-in 870P, a used Wingmaster, or even an older pre- plastic trigger plate/magazine tube dimple/MIM parts Express gun for a couple hundred bucks or less, then spend the balance on getting it exactly precisely how you want it to be. Robar indeed does good work, Lee Hadaway (used to pull the freight at Scattergun before they sold out to Wilson) is still going good 870 work too (
http://www.thearmsroom.com/_wsn/page10.html ), Tank's Rifle Shop has a certain following too. Any number of local 'smiths can build excellent 870s as well.
If you just gotta gotta gotta have the Wilson, go ahead and buy it. But it is too much sugar for a cent, and IMHO you can actually get a better gun for less money by pursuing other avenues.
Stay safe,
lpl/nc