The newer guns say "Remington 870 Magnum Police" on the left side of the receiver.
The older police guns were marked simply as Wingmasters, and were just Wingmasters with a satin blue or parkerized finish, and an oil finished, un-checkered stock.
You can usually ID a Police gun, new or old by the shorter police-length forearm with it's grooved gripping surface, and the un-checkered stock with it's dull oil finish.
Commercial guns have the longer, checkered fore end and the checkered stock, usually with the shiny "Bowling pin" finish.
Both the Wingmaster and the Police receivers are better quality than the Express, with a smoother finish, and no burrs or machine marks.
For your project, you can use either a genuine Police model or a Wingmaster.
A Wingmaster is fairly easy to find, much more so than a good used Police-marked gun.
Since you intend to replace most everything other than the receiver, a Wingmaster is will do nicely, except it won't have the word "Police" on the side.
Price-wise, Wingmasters in good condition are usually cheaper and easier to find than Police-marked guns in the same shape.
Bottom Line: The Express is an 870 with a down-graded finish and some plastic parts to lower the price to compete with the cheaper to make Winchester and Mossberg guns.
The Wingmaster and the Police are basically the same gun.... One with a shiny commercial finish, one with a dull finish.