Remington makes the Express to sell for a lower price, hence they have to build it with less costs. To set up and build all different small parts would hardly be conducive to that end. Look at the parts list, the magazine spring, just as an example, is the same for the Express as for the others. The extractors are not all the same - MIM versus milled. What they do mainly is less finishing and polishing and woodwork on the Expresses.
I would not buy an Express and try to make it into a Wingmaster. Unless you own a machine shop and have gunsmithing facilities (hot blue, etc.), you cannot substitute your labor for dollars. Makes no sense unless you just want to see if you can do it. With enough money, you can do anything. Well, almost. You can't be 25 but once.
I do not think you can beat a used Wingmaster for value. There are just sooooo many of them out there it keeps the prices down. I recently bought a used '76 vintage 3" Wingmaster in 95% plus condition for $240 delivered, and then got a LC RemChoke barrel for it for another $150. The wood was actually about 99% with no chips or cracks. I did have to hunt for awhile to find just what I wanted, and I would have gone another $75 or so, if I'd had to. I could sell the old barrel for probably $100, but I like that old fixed choke for trap.